IMMUNOLOGY

Immunology is simply defined as the study of how the immune system of a living organism functions in either a disease condition or a healthy state. It is the study of how an organism responds to foreign bodies (antigens) that it comes in contact with in the environment, and its differentiation from the body’s own self. The immune system is generally a complex system of network of cells, tissues and organs and cell products (e.g. antibodies and cytokines) which work cooperatively to protect the body from invading pathogenic microorganisms and the disease/infection that they cause. Susceptible individuals can be immunized through the process of immunization or vaccination, a process of injecting a person with an antiserum or immune system booster (e.g. vaccines) in order to give the body immunity or protection from an infection or infectious disease agent.

The phrase immunology also encompasses all the biological, chemical, and physical properties of an organism’s immune system that help it to fight against infectious diseases. In other words, immunology deals basically with the study of our body’s protection from foreign agents or antigens (inclusive of pathogenic microorganisms) that invade the host’s body to cause infection or disease. The environment in which we live is inundated with diverse foreign bodies or macromolecules which are generally termed antigens.

An antigen is a molecule or substance that is foreign to the body (i.e. it is unknown by the host immune system). Molecules which are unfamiliar to the host’s body and which the immune system sees as potential harm are known as non-self while those molecules, cells, tissues and organs that are valid parts of the body are called self. To be functional and active, an immune system must be able to distinguish self molecules from non-self molecules. These non-self molecules are infectious agents or particles that invade the human body and cause disease, and they include virus, fungi, bacteria, parasites/worms or protozoan and every other molecule that should not naturally be found in the human body.

The field of immunology has had tremendous success in clinical/human medicine in the area of containing the excesses of infectious diseases, and this can be envisaged in the eradication of some diseases (e.g. smallpox) which used to be a burden to humanity. Today, there are plethora of vaccines that help to prevent the acquisition and transmission of infectious diseases or their agents within a defined human population through proper immunization/vaccination. Prior to birth, the foetus in the womb of an expectant mother is usually free from microbes or foreign bodies because of the germ-free environment provided by the uterus.

But immediately after birth, the newborn becomes bathed with different types of infectious agents including microorganisms that constantly attack the body from childhood to adulthood. As earlier said, mankind live in potentially harsh environment that is packed with different kinds of microorganisms (both pathogenic and non-pathogenic) that can cause disease, but the human body has evolved mechanisms that allows it to evade the excesses of these microbes. This mechanism that helps the host’s body to be free from foreign agents is known as the immune system.

The immune system of a human host helps to establish a germ-free state (known as immunity) against invading microorganisms. Immunity which is derived from the Latin word “immunis” meaning “exempt or freedom from a burden” is a state of the host’s body that is characterized by the body’s ability to remove or counteract any trace of non-self (antigens) that enters the system. An immune state can also be achieved in a human host through immunization – during which the host’s body becomes more responsive and prepared to work against foreign molecules as they come in contact with the body. If truth be told, the science of immunology is centered and built around immunity – since this branch of biomedical science deals with how the host body is being protected (either naturally or artificially) from infectious agents.

The immune system is indeed a significant security of the human host since the immunity it provides helps a great deal to help keep the body safe from the myriad of pathogens that surrounds the environment. This system which learns and recognizes pathogens prior to and after the first attack, has evolved to mass produce cells and molecules with which it uses to eliminate a variety of pathogenic microorganisms. Of particular interest is the ability of a host immune system to produce antibodies and other receptors that are unique and specific for each of the different pathogenic microorganisms that invade the human body.

Antibodies are protein molecules produced by the immune system on exposure to an antigen, and that can combine specifically with foreign molecules. The immune system performs two basic functions in the body and these are: the function of recognition and the function of response. Following the invasion of the body by pathogenic microorganisms, the immune system recognizes these foreign agents (non-self) and differentiates them from one another, and the system also goes further to differentiate the non-self from the body’s own molecules (i.e. self molecules). After doing this, the immune system recruits and engages a variety of immune system cells and molecules (known as effector cells) to specifically respond and eliminates the antigen.

This initial attack of the host body by antigens leaves behind memory cells which help the immune system to recognize similar pathogen in a second attack, and thus direct a more robust and fast immune reaction that get rid of the antigen and prevent the development of an infection in the host. Thus when the body’s immune system develops immunity to a specific antigen or pathogen, it will continue to remain free of infection caused by that particular pathogen for a lifetime. A properly functional immune system is an asset to the body because such functional immune system will constantly police the body for the presence of antigens and other foreign bodies (inclusive of microbial pathogens) in order to different the invading antigens from the host’s self tissues and cells (Figure 1).

Figure 1: Schematic representation of the human immune system. Adapted from: http://www.livescience.com/38028-how-the-human-body-s-immune-system-works-infographic.html; accessed on 7-02-2014.

Once identified, the body’s immune system mounts an immunological attack with its associated cells and organs to ward-off and contain the nefarious activities of the invading disease-causing agent. The spleen, white blood cells (WBCs), antibodies, the thymus and the lymph nodes are some of the organs and cells of the immune system that help to protect the body from disease and disease-causing microorganisms. To ward-off antigens and possibly keep pathogenic microorganisms at bay it is critical that the human host maintains a good nutrition and ensures that diets are taking in their balanced forms inclusive of all the necessary nutritional requirements required for normal growth and body development. This is vital because some infectious diseases of man (inclusive of immunodeficiency diseases) have been linked to malnutrition.

Children and the elderly with immature immune systems are often at risk and undernourishment makes these individuals to be more susceptible to infections caused by microbes. Maternal undernourishment could also affect the immune system of the neonates as well as their birth weight and expectant mothers are always advised to maintain good nutrition for both their health and that of the unborn child. And even in a diseased state, the loss of nutrients from the body in some disease conditions (e.g. diarrhea and vomiting) weakens the individual’s immune system and proper replacement or intake of good diet is advisable in such scenarios. Malnourished individuals (especially children and the elderly) have higher morbidity and mortality rates to infectious diseases compared to people who eat well and this also affects the course of infections in the former since their immune system have been impaired from protecting the body against invading pathogenic microorganisms and other disease causing particles or antigens.

BRIEF HISTORY OF IMMUNOLOGY

Man’s interest in achieving some level of resistance to diseases and their causative agents (i.e. immunity) gave impetus to the study of immunology. The field of immunology actually began when it was observed that people who had earlier suffered and recovered from a particular infection or disease were afterwards discovered to have been protected from that disease. These individuals after contracting, suffered and recovered from certain diseases were no longer susceptible to the same disease or its causative agent upon a second recurrence of the malady. This singular serendipitous discovery that people were immune to certain diseases after contracting and recovering from it paved the way for the development of this very important discipline of the biomedical sciences, immunology.

In 430 BC in Athens, Thucydides (an historian of the Peloponnesian war) discovered during the plague of Athens that only people who had earlier on suffered and recovered from the disease outbreak could care for those suffering from the plague without contracting the disease a second time. Since then, several attempts have been made by man to stimulate an immune state in order to keep pathogenic microorganisms and their causative agents at bay. In particular, the Chinese used a particular technique called variolation to inoculate individuals with dried particles obtained from the smallpox (variola) boils of patients infected with variola.

Variolation is the inoculation of live microorganisms of smallpox obtained from diseased boils or abscess of smallpox infected patients who were recovering from the disease. The dried particles from the smallpox boils were either inhaled or injected into the skin of people, and this technique (variolation) helped to prevent the spread and contraction of smallpox disease as at the time. Edward Jenner in 1798 built on the successes of variolation to establish a relationship between smallpox and cowpox (a milder disease than smallpox, and which affected the udder of cows). Jenner observed that milkmaids who contracted cowpox through their handling of diseased udders of cows were protected from smallpox (a fatal and disfiguring disease), and this led him to postulate that inoculating individuals with particles or fluids from cowpox boils might ultimately protect them from contracting smallpox infection.

Edward Jenner’s assumption was confirmed when he immunized an eight year old boy with fluids from cowpox pustules, and later deliberately infected the boy with smallpox after his recovery from cowpox infection. The eight year boy was immune to smallpox infection as he did not fall sick to the smallpox virus inoculation he received. Though the mechanism of protection from smallpox infection through cowpox inoculation was not understood as at the time, Edward Jenner’s work paved way for the development of vaccination/immunization which is now currently being used in clinical medicine to prevent more deaths, disability and alleviate the sufferings of patients from infectious diseases. Jenner’s work on cowpox/smallpox inoculation clearly established the fact that the human body (immune system in particular) could be stimulated on purpose to produce an immune state that will protect it from specific infectious agents and diseases.

Edward Jenner’s work was taken a little bit further by Louis Pasteur who hypothesized that the weakened or attenuated form of a pathogen (known as vaccine) is capable of establishing an immune state when injected into a host. Louis Pasteur formulated the name vaccine (derived from the Latin word vacca meaning cow) in honour of Edward Jenner’s work on cowpox and smallpox disease. Pasteur went on to develop the rabies vaccine which is used to vaccinate and protect people beaten by rabid dogs. A handful of vaccines are currently available in the market and many others are being developed still, to provide humans and animals from infectious agents and diseases. With the discovery of phagocytosis and cellular immunity by Elie Metchnikoff and other notable immunological discoveries which further helped scientists to understand immunological concepts, immunology as a field of biomedical science has contributed significantly from time immemorial even till date in alleviating the plights of patients as it relates to infectious diseases.

Currently, the field of immunology has grown from infancy to more robust 21st century techniques which are all geared towards the prevention and eradication of infectious diseases. Though pathogenic microorganisms in their own ingenuity are continuously evolving novel techniques of dodging innate immune attack and vaccination attempts, immunology still holds the potential to contain a whole lot of infectious diseases as it did to smallpox – which was successfully eradicated in 1979 through vaccination programmes. Serology, immunobiology and immunochemistry are other aspects of immunology which are being investigated to ensure improved health condition for man and his animals. In its entirety, the field of immunology has significantly improved the diagnosis and treatment of infectious diseases across the world.

RESPONSES OF THE IMMUNE SYSTEM

The immune system is a complex system which consists of collection of cells, organs, tissues and molecules that work together to protect an organism or animal from disease and invading foreign agent’s (i.e. pathogens). For the immune system to function properly and constantly keep the integrity of the human body intact and free from microbes, it must identify non-self from self and then mount an immunological attack on the non-self molecules. With its associated cells, organs and tissues, the human immune system is capable of mounting targeted and specific immunological response against any invading pathogen and other foreign bodies that gain entry into the body. All living organisms including humans are subjected to constant attack by disease causing organisms (pathogens) due to the hostile environment in which they live, and it is the job of the immune system to continually protect the body from pathogens and ensure that it is disease-free as much as possible. Because there are an increasing number of pathogens that longs to gain entry into the intact human body, the immune system constantly evolves its mechanisms to better detect and respond appropriately to foreign bodies.

Upon detecting an antigen (which could comprise of a bacterium, fungus, virus or protozoan), the immune system uses its cells and associated structures to properly distinguish them from the normal healthy tissues and cells of the host. It is noteworthy that the actual components of the immune system are cellular in nature and not necessarily associated with any specific organ or tissue. The immune system components are widely present in the blood and fluid circulation throughout the body of every animal host (inclusive of humans). The immune system of animals is amazingly powerful in that it can recognize specifically countless number of pathogens that attack the body and cause disease, and in its sole task of ensuring an immune state of the host’s body, it in-turn produce specific molecules that wade-off the deleterious effects of the antigens. However, diseases can emanate when and if the immune system becomes disrupted or malfunctions; and this could result into the development of diseases collectively known as autoimmune diseases or disorders (e.g. rheumatoid arthritis and systemic lupus erythematosus amongst others).

Autoimmune diseases are diseases caused by an attack of the immune system on an individual’s own body tissues (i.e. self-molecules); and diseases that arise due to the malfunctioning of the immune system is highlighted in the later part of this Chapter. The immune system is thus the natural defense of combating diseases and microbial invasion of the human body but when this natural mechanism of fighting infectious diseases becomes compromised (e.g. in HIV/AIDS infection) or fails to regulate its immune responsiveness especially in the discrimination of self-molecules from non-self molecules (i.e. antigens) several untoward effects including immunodeficiency diseases and autoimmune diseases occur in the host. The primary function of the immune system in a living organism is to distinguish between self and non-self molecules or foreign bodies. In its task of protecting the body from infectious agents and safeguard it from the development of disease, the immune system responds by mounting series of reactions known as immune response – which collectively develop immunity (resistance) to only non-self molecules without attacking the body’s own molecules recognized as self.

The immune system responds in two basic ways to non-self molecules or antigens, and these are:

  • Innate immune response: The major purpose of the innate immune response after pathogen invasion of the body of a host is to attract immune system cells such as antibodies to the site of an infection in the body. And once this has been established, the adaptive immune response (which is more specific in nature than the innate immunity) is attracted to the site of infection and stimulated to produce targeted immune responses that inhibit or kill the invading pathogen or antigen; and thus restore the host to its normal body function.
  • Adaptive immune response: Adaptive immune response as shall be seen later in this Chapter is antigen-specific; and it recognizes and destroy a particular antigen in a host especially based on previous exposure of the host to the invading pathogen(s). However, both the innate and adaptive immune response works together in our body system to mount an effective and specific immune response upon pathogen exposure or invasion.

The resistance of the body to disease and foreign bodies is usually based on these two mechanisms of immune response i.e. the innate and adaptive immune response. The immune response of the immune system recognizes antigens and immediately mounts a biological reaction to eliminate it while helping the system to recognize it and mount a rapid attack the next time similar foreign agent invades the body.

Also, the adaptive immune system of animals can be divided into two groups as follows:

  • Humoral immunity or antibody-mediated immunity (AMI)
  • Cell-mediated immunity (CMI)

Both AMI and CMI shall be highlighted later in this section.

OVERVIEW OF ANTIGEN

Antigen is any substance or molecule that can trigger an immune response in an animal. An antigen is basically anything that is foreign to the body and which can react specifically with an antibody. An understanding of the basic characteristics of antigens and/or pathogens that spark immunological response in the body is vital for us to know how antigen-antibody reaction actually occurs and what this complex reaction implies in clinical terms. Antigens consist of microorganisms such as viruses, bacteria, fungi and parasites or worms. They may also include proteins, glycolipids, polysaccharides, and substances or molecules released by any of these pathogens, and which the immune system of the host organisms considers to be non-self molecules.

In transplantation immunology where graft or organs are being transplanted from one host to another, the recipient immune system (especially in the case of non-identical twins) can sometime see the transplanted tissues or cells as antigens because they may contain some markers which the non-self receiver’s immunological makeup sees as non-self. However, not all foreign molecules that enter the body are capable of sparking up an immunological response that leads to antibody production.

Antigens that induce the production of antibodies by the immune system are generally called immunogens. All immunogenic substances are always antigenic because they are able to react and be recognized by a specific antibody. However, some antigens (especially those with low molecular weight) are not immunogenic in nature even though they may exhibit some features of an antigen and are said to be antigenic. Substances with low molecular weight and which are not immunogenic by themselves but can become immunogenic when coupled to a carrier molecule such as a protein (which is immunogenic) are generally called haptens.

Drugs can sometimes become immunogens when they spark up allergic reactions in the host taking them. Many biologically and chemically important substances such as drugs, steroid hormones, and peptide hormones can also serve as haptens. Dinitrophenol (DNP), an organic compound is a typical example of a hapten. The phrases antigen and immunogens are often used interchangeably. Immunogenicity is the ability of a substance to elicit an immune response (both humoral and cell-mediated immunity), and this phenomenon is usually exhibited by immunogens.

Antigenicity is the property of an antigen that allows it to react specifically with the product of the specific immune response (i.e. antibodies or receptors of T cells). Though related and often confused in immunological discussions, immunogenicity and antigenicity are two distinctive terms which varying immunological properties and functions as it relates to antigens. In immunological terms, antigens are generally referred to as substances that can react specifically with the antibody receptor of B cells or T cell receptor when complexed or joined with major histocompatibility molecule (MHC). Antigens are of two types, and they are endogenous and exogenous antigens.

1. ENDOGENOUS ANTIGENS

Endogenous antigens are naturally occurring antigenic substances found within an animal and which are unique to every individual. They are generally responsible for tissue rejection experienced during the transplanting of cells or tissues from one host (donor) to another (recipient). Typical examples of endogenous antigens are allogenic antigens which are largely controlled by antigenic determinants that differentiate one individual of a species from another. They are specific cell markers found on the surfaces of cells and tissues and are seen as antigens by the immune system when they move from one person to another in scenarios of blood transfusion or tissue transplant.

2. EXOGENOUS ANTIGENS

Exogenous antigens are foreign molecules or substances that are external to the body of a host. They include microbes and their products, proteins of high molecular weight and polysaccharides as well. Exogenous antigens are responsible for a handful of infectious diseases in man, and they elicit both specific and non-specific immunological attack upon entry into the body of a host.

CHARACTERISTICS OF ANTIGENS

However antigenic a foreign molecule/substance may be, to be immunogenic and qualify to be called an actual antigen to the extent of eliciting the host’s immune system to mount a specific immune response or attack, antigens must meet certain criteria that actually make them eligible immunogens (i.e. B cell and T cell activators). The immune system of animals actually recognizes microbes and their products as immunogens ever before it can protect the host from infectious diseases that they cause. When an antibody recognizes and binds to a specific site (usually the epitope) on the antigen, and antigen-antibody complex is formed, and this phenomenon leads to the destruction of the immunogen (Figure 2). The characteristics of antigens that determine their immunogenicity are highlighted in this section.

1. EPITOPES (ANTIGENIC DETERMINANTS)

Epitope is the discrete site on the structure of an immunogen that is recognized by an antibody. It is those areas on the surface of an immunogen that stimulate specific immune response in a host. Epitopes are the recognition and binding sites of an antigen that bind to antigen-specific receptors or secreted antibodies. Antigens bind to antibodies with the help of some intercellular forces such as Van der waals forces, hydrogen bonds and hydrophobic or electrostatic forces. Antibodies that fit and bind closely with antigens without dissociation are said to have high affinity. Affinity is the binding strength between a single receptor site on an antibody and a single epitope on an antigen. It is simply the strength of binding of an immunogen to an antibody. The overall strength of binding or interaction between an immunogen with many epitopes and a polyvalent antibody (e.g. IgM) is known as avidity.

Immunogens with multiple antigenic determinants are generally known to elicit various types of antibodies as well. As immunologically active regions, an antigen or immunogen can have many different antigenic determinants (epitopes) which are specifically recognized by both the B cells and T cells. Most immunogens are proteins in nature, and thus exist as a folded, three-dimensional tertiary structure that consists of amino acid clusters containing a number of antigenic determinants to which antibodies bind to. A particular microorganism can have several epitopes to which antibody binds. For example, on the surface of a bacterium are found cell wall (O) antigen, capsular (vi) antigen, flagella (H) antigen, and a host of other antigenic sites that antibodies binds to specifically.

 

Figure 2: Illustration of antigen-antibody complex formed during the body’s invasion by foreign bodies (e.g. pathogenic microorganisms). Adapted from: http://www.livescience.com/38028-how-the-human-body-s-immune-system-works-infographic.html; accessed on 7-02-2014.

2. MOLECULAR WEIGHT/SIZE

The molecular size or weight of a microorganism or an immunogen is important in determining the molecules immunogenicity. For a foreign substance to be immunogenic, it must be of certain molecular size that is capable of eliciting an immunological response. Usually, antigens less than 10,000 Daltons in their molecular weight are said to be weakly immunogenic. Immunogens have molecular weight that is above 10,000 Daltons. Non-immunogenic substances such as amino acids are of a molecular size lesser than 1000 Daltons.

3. FOREIGNNESS

Foreignness is the first and most important characteristic of an antigen because for any substance to be regarded as an immunogen it must first be genetically foreign to the host cells. The immune system of a host must first recognize a substance as non-self upon entry into the body for it to spark an immune response. However, the greater the phylogenetic difference between the non-self and self molecules, the greater the structural difference between them and the greater the degree of its foreignness too. Self or host molecules are generally non-immunogenic while non-self molecules are recognized as antigens and are immunogenic. Though the human immune system professionally discriminates between self and non-self molecules, it must be understood that in some physiological state of the body, some self molecules may be seen as non-self molecules and this can lead to a disease conditions known as autoimmune disease.

4. CHEMICAL COMPOSITION AND HETEROGENEITY

The chemical makeup and heterogeneity or complexity of a foreign substance is essential to that antigen’s immunogenicity. To be considered an immunogen, antigens must attain some level of structural complexity i.e. they must be made up of different components instead of being composed of only just one part. The complexity and conformation of an immunogen are determined by both the physical and chemical properties of the molecule. Monomeric/homopolymer protein molecules (i.e. protein molecules with only one composition) are less immunogenic than polymeric/heteropolymer proteins consisting of the primary, secondary, tertiary and quaternary structures of a protein molecule. Thus, antigens become better immunogens when they are composed of several amino acids and sugars bonded together. The immunogenicity of an antigen can also be enhanced especially when they are coupled to substances that help to increase their structural complexity and thus their molecular size as well. Such substances that increase the immunogenicity of an antigen and thus enhance immune response to them are known as adjuvants. Some examples of adjuvants include saponins, calcium salts, aluminium salts, and lipopolysaccharides (LPS) amongst others. Adjuvants are generally structurally different compounds linked together to increase immune response to an antigen.   

5. GENETIC MAKEUP OF THE HOST ORGANISM

The genetic makeup of an organism to some extent determines the immunogenicity of an immunogen. The level of antibody production and T cell response to an immunogen may vary across species of an organism. In other words, two strains of the same species of an organism may respond in different ways to the same immunogen due to variation in the genetic makeup of their immune system. No matter how potent the immunogenicity of an immunogen is, its capacity to elicit an immune response in a host will largely depend on the biological composition of the individual’s immune system. The route via which the immunogen entered, its dosage and the genotype of the host are all factors that the immunogen must contend with for it to elicit an immune response. Immunogens administered parenterally sparks up rapid immune response than those introduced via the alimentary canal or gut, and the amount of immunogen administered is also crucial as the higher the quantity of immunogen the higher the degree of immune response. The administration route of an immunogen however, determines the type of immune system cells and organs that will be involved in eliciting an immune response.        

6. SUSCEPTIBILITY TO ANTIGEN PRESENTING AND PROCESSING CELLS (APCS)

Immunogens must be properly processed by antigen presenting cells (APCs) and be presented on MHC molecules to T cell receptors for an appropriate development of adaptive or acquired immunity. An interaction between T cells with immunogens that has been processed and presented together with MHC molecules is crucial for the development of cell-mediated and humoral immune response. Upon entry into the body of a host, an immunogen must first be degraded and presented with MHC molecules by APCs for immunogenicity to take place. Immunogens that are large (i.e. with larger molecular size) and insoluble are strongly immunogenic than small soluble molecules because the larger macromolecules or immunogens are more readily engulfed by phagocytes and processed by APCs for immunogenicity to take place.

OVERVIEW OF ANTIBODIES

Antibodies are soluble protein molecules produced by the B cells of the immune system in response to a specific antigen. They are antigen recognition molecules; and antibodies generally recognize and bind to specific antigens. The term antibodies will be used synonymously with immunoglobulins (Ig) in this textbook. Antibodies are glycoproteins that are produced by B cells (specifically plasma cells) in response to an immunogenic substance. After production by the B cells, antibodies continue to circulate in the bloodstream as effector cells of the antibody mediated immunity (AMI). Other antigen recognition molecules include major histocompatibility complex (MHC) molecules, T cell receptors, and the B cell receptors. The production of antibodies is stimulated foreign bodies or antigens that enter the body. Immunoglobulins or globulin proteins are mainly found in blood plasma where they unleash their immunological attack in the face of an antigen. Antibodies perform various functions in the body and these shall be highlighted in this section.

  • Antibodies act as opsonins to facilitate the process of phagocytosis. This process is known as opsonization; and it is the deposition of opsonins on the surfaces of pathogenic microorganisms or antigens so that they can be readily phagocytosed by phagocytic cells.
  • Antibodies known as agglutinins react specifically with antigens to form clumps in the process of agglutination. This is the basis for most of the in vitro latex agglutination test performed in the laboratory. Agglutination is the visible clumping of particulate antigens by antibodies.
  • Antibodies help to activate nonspecific immune response against an invading pathogen.
  • Antibodies help to activate the complement system after binding with a particular antigen; and this process stimulate opsonization and microbial cell lysis. The complement system is a system that consists of a series of serum proteins that is activated by antigen-antibody complex to facilitate phagocytosis and other immunological reactions.
  • Antibodies serve to mark and identify specific targets for immunological attack in vivo.
  • Antibodies search the bloodstream for antigens and neutralize them or mark them for elimination by other cells of the immune system (e.g. phagocytes).
  • Antibodies promote antibody-dependent cell-mediated cytotoxicity (ADCC). ADCC is a cell-mediated reaction in which nonspecific cytotoxic cells that expresses Fc regions (i.e. crystallizable fragment portion of an immunoglobulin) such as macrophages and natural killer (NK) cells recognize and bind antibody on a target cell and subsequently cause the lysis of the target cell.
  • Antibodies neutralize pathogenic microorganisms (inclusive of bacteria, toxins and viruses) in the process of neutralization.

The main structure of the immunoglobulin or antibody is a monomer that resembles a Y-shaped-structure with two arms and a tail or stalk region (Figure 3). An immunoglobulin (Ig) or antibody is made up of four polypeptide chains that comprise of two identical heavy chains (H) and two identical light chains (L) that are covalently linked by disulphide bonds (Figure 3). Each of the heavy chains and light chains also have constant (C) and variable (V) regions which varies amongst the two chains; and the polypeptide chain of Ig have amino terminal and carboxyl terminal ends which form part of the V and C regions respectively. For example, an L chain usually consists of one variable (VL) domain and one constant (CL) domain. However, the H chain of some antibodies may consist of one variable (VH) domain and more than one constant (CH) domains.

Structurally, antibodies have two main regions or fragments which are the antigen-binding fragment (Fab) and the crystallizable fragment (Fc). The Fab region is the fragment or region of the immunoglobulin that specifically binds antigens at their epitopes or antigenic determinant sites while the Fc region is the fragment that activates complements and phagocytosis. The Fc region is the site at which the immunoglobulin molecule binds to a cell, and this region lacks the affinity or ability to bind antigens. It is noteworthy that the Fab fragments have both constant and variable regions while the Fc fragment has only the constant region. Each H chain and L chain in an antibody consist of an amino-terminal variable (V) region that is made up of several amino-acids (ranging between 100-110 amino acids); and these amino acid sequence that line the H and L chains of Ig are largely responsible for the specificity of the antigen binding site of the antibody. The amino terminal region of the H and L chain varies greatly among antibodies. The hinge region separates the two antigen-binding sites; and it is the region of the heavy chains between the constant domains held together by disulphide bonds. The hinge region is flexible in IgA, IgD and IgG but inflexible in IgM and IgE.

Figure 3: Basic structure of an antibody (a). The three-dimensional arrangement of an antibody structure is also shown (b). The binding of antigen to antibody is like a lock and key mechanism. The Fab region is mainly responsible for binding to antigens while the Fc region apart from binding to cells, also help to determine the isotypes of immunoglobulins. Source: David S, David M.H, Craig H.H and May B (2010).  Puvet/Sadava//Orians/Heller Life editions: The Science of Biology, 8th edition by Sinauer Associates, W.H. Freeman and Company, Sunderland

TYPES OF ANTIBODIES

Immunoglobulins do not actually kill or eliminate pathogenic microorganisms or antigens from the body. The role of antibodies during an infection is to bind specifically to invading pathogens so that the antigen-antibody complex so formed can be made readily available for other immune system action such as opsonization, phagocytosis, neutralization or complement fixation which ultimately spurs into action to kill and eliminate the bounded antigen from the system. And this is how the antibody-mediated effector function operates. Antibodies or immunoglobulins have various isotypes and these shall be highlighted in this section. These isotypes or classes of immunoglobulins are known as isotypes because they are variants of the main immunoglobulin molecule (i.e. immunoglobulin G). And each of these isotypes of classes or immunoglobulins is differentiated by their distinctive amino acid sequences which are known to line the heavy chain constant region of the antibody molecule.

The different immunoglobulin isotypes also have different subclasses which vary in different animal species. Differences in the heavy chains of each of the immunoglobulin isotypes are usually used to characterize the different Ig subclasses. Human beings for example have four subclasses of immunoglobulin G (IgG) viz: IgG1, IgG2, IgG3 and IgG4. Immunoglobulins also have allotypic determinants and idiotypic determinants aside the different isotypes of antibodies known. The allotypic determinants also known as allotypes are antigenic determinants mainly found in the constant regions of immunoglobulins. The idiotypic determinants or idiotypes are antigenic determinants found in the variable regions of antibodies. The five major classes or isotypes of immunoglobulins are differentiated from each other by the type of heavy (H) chain found in the antibody molecule.

  • Immunoglobulin A (IgA): IgA has alpha (α) heavy chain.
  • Immunoglobulin M (IgM): IgM has mu (µ) heavy chain.
  • Immunoglobulin D (IgD): IgD has delta (δ) heavy chain.
  • Immunoglobulin E (IgE): IgE has epsilon (ε) heavy chain.
  • Immunoglobulin G (IgG): IgG has gamma (γ) heavy chain

IMMUNOGLOBULIN A (IgA)

Immunoglobulin A (IgA) is an antibody found in serum (as a monomer of about 150,000 Daltons) and in body secretions (as a dimer of about 600,000 Daltons); and it is mainly produced in the body by mucosal associated lymphoid tissue (MALT). Though it mainly exists as a monomer and dimer (Figure 4) IgA can also exist in polymeric forms. It is mainly found in external mucosal secretions of the body such as in the secretions of the GIT, lungs or bronchial secretions and the genitor-urinary tract. Saliva, tears, colostrum or breast milk, sweat and nasal fluids are other external body secretions in which IgA is found. The immunoglobulin A of external body secretions (e.g. saliva, tears and breast milk) is generally known as secretory immunoglobulin A (sIgA). sIgA is produced mainly by epithelial cells of secretory membranes; and they are usually the first line of defense against pathogenic microorganisms (inclusive of bacteria and viruses) that invade the mucosal surfaces of the body. For example, the mucous membrane surfaces especially those found in the nasal areas, GIT, genitor-urinary tract, mouth and eye region are usually the main portal of entry of some disease-causing agents such as bacterial and viral pathogens.

The main biological function of sIgA at these mucosal surfaces is to prevent the attachment of pathogens to the mucosal surfaces by specifically binding to viral and bacterial surfaces antigens. After binding, the mucous entraps the IgA-antigen complex formed; and this complex is later eliminated from the body via the ciliated epithelial cells and peristalsis mechanisms found in these mucosal surfaces. The presence of sIgA in the breast milk of infants help to protect the newborn from infectious disease and/or pathogens in their first few months of life since their immune system is not fully developed and functional. The type of immunity provided by sIgA in the breast milk of infants is known as passive immunity since this type of protection only last for a short period of time (e.g. about six months). At the mucosal surfaces, sIgA and IgA in general prevents the adherence of coated pathogenic microorganisms (e.g. bacteria and viruses) to the surfaces of mucosal cells, and thus prevent their entry into internal body tissues and into the circulatory system. IgA does not cross the placenta and it activates the complement system only via the alternative pathway.

Figure 4: Illustration of immunoglobulin A (IgA). IgA is generally a monomer (left). The illustration on the right-hand side is a schema of secretory IgA (sIgA), a dimer; and it is usually found in external body fluids or secretions such as breast milk and saliva and other internal body fluids. Immunoglobulin A protects mucosal surfaces of the body; and sIgA is the main antibody found in external secretions such as intestinal mucous, tears, saliva and respiratory and genitor-urinary tract fluids. Immunoglobulin A is also the main protective antibody in the breast milk (e.g. colostrum) of infants where it provides a passive type of immunity.

COLOSTRUM

Colostrum is the first secretion of the mammary gland (breast) produced before proper lactation commences in a nursing mother. It is the first breast milk produced from the mammary gland or breast of a woman after giving birth to a baby. It is a major source of passive immunity transferred from mother to the newborn child; and colostrum confers several health benefits to the newborn inclusive of growth factors or nutrients aside its protective function in the immune system of the host. Colostrum is generally the first milk produced from a female animal inclusive of humans after 48 hours of delivery; and this milk is characteristically thick and yellowish in colouration.

The main immunoglobulin molecule found in the colostrum is immunoglobulin A (IgA) or secretary immunoglobulin A (sIgA). After birth, the newborn is exposed to plethora of microorganisms in the environment including those acquired from the vagina or birth canal of the mother; and since the immune system of the infant is not well developed and strong to fight against pathogens, it is the immunologic defense provided by the colostrum that help to protect the child from harmful activities of microbes it comes in contact with. Though this type of immunity is passive in nature and does not last long; it serves as the major basis for the development of the newborn’s own immune system at a later stage in life.

The production of colostrum in the mammary gland of humans and other animals usually continues through the early few days or weeks of delivery after its production in the last phase of pregnancy or after delivery. Nevertheless, the production of colostrum lasts for about 2 – 4 days after the lactation phase has begun in the mother. It is advisable according the World Health Organization (WHO) that mothers should ensure that their newborns have unrestricted access to this milk, and that breastfeeding should be “exclusive” since the breast milk from the mammary gland contains the major nutritive requirement for the development of the child inclusive of water for about 6 months. However, this practice of “exclusive breast feeding” is not evenly practiced in some cultures due to some religious, occupational and cultural believe of some ethnicity.

Colostrum is rich in carbohydrates, amino acids, proteins, and antibodies; and it is generally low in fats. It is also rich in vitamins and other minor and major minerals required keeping the newborn healthy. Colostrum gives a general immunity (though passive in nature) to the newborn from the mucous membrane of the mouth or throat, down to the lungs and even the gastrointestinal tract (GIT) – where it protects the infant from some GIT-related microbial infections such as diarrhea. Breast milk is still the safest and healthiest from of food for a newborn; and it is advisable that mothers exclusively breast feed their babies for six months after which they can be weaned and given other forms of food.

IMMUNOGLOBULIN M (IgM)

Immunoglobulin M (IgM) is an antibody that mainly exist as a pentamer i.e. it consists of five different monomeric units joined together by disulphide bonds and a joining (J) chain (Figure 5). The function of the J chain in the IgM structure is to help in the polymerization of the individual monomeric units that makeup the pentameric IgM antibody; and it does this via a sulphydryl residue close to the carboxyl terminal of the Fc region of the monomeric units. It is the largest antibody out of the 5 isotypes of immunoglobulins; and IgM is often known as a macroglobulin because of its relatively higher molecular weight or size which is about 900,000 Daltons (Da). Because of its large size, IgM does not cross the placenta and it does not leave the blood stream where it gives the host protection against blood-borne infections (caused by both bacteria and viruses). Though the large size of IgM confines the antibody to the bloodstream where it provides protective functions against pathogenic microorganisms, IgM has a short lifespan and it is the first antibody to disappear from the blood serum.

Immunoglobulin M is the main antibody produced in primary immune response to an invading pathogen or antigen. IgM is also the first antibody to be produced in neonates or infants; and immunoglobulin M (a pentameric antibody) is the most efficient complement fixing antibody. This is because it has ten (10) antigen-binding sites unlike other isotypes of immunoglobulins; and IgM is more efficient in binding antigens with many antigenic determinant sites or epitopes (e.g. red blood cells and viral particles). Thus IgM is a better complement fixation antibody, and it is very efficient in agglutination and in the cytolysis of microbial cells (e.g. bacteria). The complement system usually requires two adjacent Fc regions for complement activation; and this is provided by the pentameric structure of IgM; and this feature makes IgM a better complement fixation antibody than the other immunoglobulins. Increase in the production of IgM usually signifies an in utero infection in expectant mothers because it is the first immunoglobulin to be produced in neonates. IgM is usually present on the surfaces of all uncommitted or naïve B cells and it is the first immunoglobulin to be synthesized during B cell maturation in the bone marrow.

Figure 5: Schematic illustration of immunoglobulin M (IgM), a pentameric antibody. Immunoglobulin M is the first antibody to be produced during infection (i.e. in primary immune response to an antigen). The J chain helps IgM to bind to receptors on secretory mucosal cells; and this promote the entry of IgM into external body secretions (e.g. tears, saliva and breast milk). Each of the monomeric unit that make up the pentameric IgM structure are arranged with their Fc regions in the center of the pentamer while the 10 antigen-binding sites are arranged at the outside or periphery of the structure. IgA is another immunoglobulin apart from IgM that has a joining (J) chain.

IMMUNOGLOBULIN E (IgE)

Immunoglobulin E (IgE) is an antibody that is known to bind to host tissue cells (e.g. mast cells), and it is largely responsible for most hypersensitivity reactions (i.e. allergy) in the body. Some of the symptoms of the allergic reactions mediated by IgE response to the invasion of antigens include anaphylactic shock, hay fever and asthma. IgE has a structure similar to the generalized immunoglobulin structure; and it is made up of two pairs of heavy and light chains that are joined by disulfide bonds (Figure 6). The type of allergic reaction mediated by IgE is known as Type I hypersensitivity reaction. Type I hypersensitivity reaction which can also be called anaphylactic or atopic hypersensitivity is an IgE-mediated type of immediate allergic reaction that provides a systemic or local protective functions at various body sites especially on the skin. Immunoglobulin E is specifically bound to the Fc receptors found on mast cells; and it controls the activities of the mucosal associated lymphoid tissue (MALT) which provides protective functions around various mucous membrane surfaces in the body (inclusive of the nasal area, oral cavity and intestinal mucosal surfaces).

Immunoglobulin E is the main protective antibody against parasites or helminthes; and they do not cross the placental barrier. However, IgE activates the complement system by the alternative pathway. IgE exist in serum at a relatively low level, and the antibody has a MW of about 190, 000 DA. The allergic reactions mediated by IgE response to the invasion of antigens leads to the release of pharmacologically active mediators (e.g. histamines, leukotrienes and prostaglandins) by the mast cells; and it is these pharmacologically active substances released by the mast cells that are largely responsible for the hypersensitivity reactions associated with IgE function. Also, the pharmacologically active mediators facilitate the release and build up of other immune system cells and molecules that are required to defend the body against parasitic infections. The reaction that goes on between the binding of IgE to mast cells and the release of vasoactive substances by basophils is shown in Figure 7.

Figure 6: Schematic illustration of immunoglobulin E (IgE). IgE has a high affinity for mast cells (e.g. basophils); and the antibody is a cell membrane bound immunoglobulin that mediates hypersensitivity reactions in the body. IgE binds to the Fc region of basophils; and this result in the release of vasoactive substances (e.g. histamines) following the degranulation of mast cells.

Figure 7: Schematic illustration of degranulation of a mast cell by IgE-mediated binding. The bridging of two IgE molecules on mast cell membranes causes the release of granules and/or vasoactive substances as shown in the illustration. IgE is mainly responsible for the mediation of immediate Type I Hypersensitivity reactions that cause hay fever, asthma, hives, and anaphylactic shock; and the immunoglobulin assist in defending the body against parasitic or helminthic infections. IgE bind with high affinity to Fc receptors on the surfaces of mast cells and/or basophils; and this binding activates the degranulation of the mast cells to release vasoactive substances inclusive of histamines, cytokines, prostaglandins and leukotrienes amongst others. These pharmacologically active substances mediates Type I hypersensitivity reactions in the body. Source: Shakib F, Ghaemmaghami A.M and Sewell H.F (2008). The molecular basis of allergenicity. Trends in Immunology, 29(12):633-642.

IMMUNOGLOBULIN G (IgG)

Immunoglobulin G (IgG) is a monomeric antibody and the most predominant immunoglobulin in secondary (memory) immune response to an invading antigen (Figure 8). It accounts for about 80 % of the total immunoglobulin pool in blood serum or plasma and tissue fluids. IgG unlike other immunoglobulins crosses the placental barrier, and thus it provides the neonate or newborns with humoral immunity during the first six months of life. Immunoglobulin G is a moderate complement fixing antibody, and it mediates antibody dependent cell- mediated cytotoxicity (ADCC). IgG has a molecular weight of about 150,000 DA.

The ability of IgG to cross the placenta gives it an additional biological function of providing naturally acquired passive immunity to neonates in utero and even at birth. IgG appears late during an infection and it persist longer in the bloodstream unlike the other immunoglobulin isotypes. The binding of IgG to pathogenic microorganisms inclusive of bacteria and viruses facilitates the opsonization and phagocytosis of the invading antigens. Microbial cells that are opsonized are easily phagocytosed than non-opsonized antigens. IgG is mainly used in immunological research because of its relative abundance in blood serum/plasma and its high affinity and specificity for antigens.

Figure 8: Schematic illustration of immunoglobulin G (IgG). IgG is the perfect model for understanding the structure and function of other immunoglobulin isotypes. VH=variable heavy chain, CH=constant heavy chain, VL=variable light chain, CL=constant light chain.

IMMUNOGLOBULIN D (IgD)

Immunoglobulin D (IgD) is an antibody with the basic four polypeptide structure of an immunoglobulin (Figure 19.9); and IgD is mostly found attached to the membranes of B cell. IgD is a membrane bound immunoglobulin, and it has a molecular weight (MW) of about 180, 000 DA. Immunoglobulin D is present in only trace amounts in serum; and in association with IgM, IgD is the main membrane-bound antibody commonly expressed on the surfaces of mature B cells or lymphocytes where they are believed to function as antigen receptors. IgD does not fix complements, and it does not cross the placental barriers. IgD is absent from memory cells, and their full physiological function in the immune system is yet to be unraveled.

Figure 9: Illustration of immunoglobulin D (IgD), a membrane bound immunoglobulin. IgD is mainly found on the surfaces of B cells as receptors.

ANTIGEN-ANTIBODY REACTION

Antigen-antibody reaction is an immunological reaction in which a particular antibody molecule reacts with a specific antigen to form an antigen-antibody complex which is marked for further immunological response by other components of the immune system. It can occur inside a living organism (i.e. in vivo reaction) and it can also occur outside the host body (i.e. in vitro reaction). The main biological function of an antigen-antibody reaction within the body of the host organism (i.e. in vivo) is to identify and mark antigens or pathogenic microorganisms for destruction and elimination from the body by other specific components of the immune system e.g. pathogen destruction by phagocytes.

Antigen-antibody reaction is a reversible immunological reaction; and though it bears a resemblance to the notable enzyme-substrate reaction, there is no apparent degradation in the chemical structure of either the antibody or antigen involved in the reaction. Immunoglobulins generally have a “Y” shaped structure that comprises of two main parts which is the antigen-binding fragment (Fab) and the crystallizable fragment (FC) which binds antigens and host cells respectively. Antibodies generally react with antigens by binding the antigens specifically at their epitopes or antigenic determinant sites using their antigen-binding fragments (Figure 10).

The complementarity determining regions (CDRs) of the antibody react with the epitopes or antigenic determining sites of the antigen(s) to form an immune complex known as antigen-antibody complex which is held together by non-covalent bonds or forces such as van der Waals forces and hydrogen bonds amongst others. CDRs which can also be called hyper-variable regions (HVRs) are specific areas within the variable regions of an immunoglobulin molecule (i.e. the VH/VL domain) as well as the TCRs of T cells that specifically bind to antigens, and thus determine the antigen-binding specificity of the molecule.

The strength of antigen-antibody binding or interaction is generally known as avidity. Avidity is defined as the functional binding strength between an antigen and antibody; and it generally reflects the interaction between the epitopes and the CDRs/HVRs of the interacting immunoglobulin molecule(s). Avidity expresses the binding power or capacity of an immunoglobulin molecule rather than its affinity i.e. its attraction to particular epitopes of an antigenic molecule. It is the ability of an immunoglobulin molecule to express multiple interactions (especially with multivalent antibodies such as IgM) with an antigen.

Figure 10: Schema of antigen-antibody reaction. The lock and key binding-mechanism that is characteristic of antigen-antibody reaction is depicted in this representation. The two ends of the Y structure of the antibody (otherwise known as the antigen-binding fragment) can be seen combining with the antigen at its antigenic determinant site or epitope to form an immunological complex known as antigen-antibody complex.

Immunoglobulins react specifically with the surfaces of pathogenic microorganisms or soluble antigens (that invade the body) in an immunological fashion; and this interaction between the antigens and the antibodies leads to the formation of an antigen-antibody complex as earlier stated. The formation of the antigen-antibody complex produces an observable clump (especially in vitro) to form agglutinates and precipitates in agglutination and precipitation reaction respectively. Precipitation reaction is an immunological reaction in which an antigen reacts with antibodies known generally as precipitins to form visible clumps or precipitates.

Antigen-antibody reaction whether in vivo or in vitro is of tremendous clinical or medical importance because the formation of antigen-antibody complexes in vivo for example makes pathogenic particles or soluble antigens to be easily opsonized and phagocytosed by phagocytes, macrophages and other components of the immune system. The immune system is stimulated to respond immediately and mount an immunological response that will result to the elimination of the invading disease agent or foreign body from the host body. In addition, the formation of antigen-antibody complexes in vivo also makes pathogenic microorganisms to be easily neutralized by other components of the immune system.

For in vitro or reaction, antigen-antibody complexes that result in the formation of visible clumps known as agglutinates in agglutination reaction or precipitates in precipitation reaction help medical scientists to unravel the causative agents of some infectious diseases; and this techniques have been widely used in some rapid diagnostic tests (otherwise known as agglutination tests for example) to detect the presence of antibodies in patients serum or plasma after an infection or disease process in the host. Typical examples of in vivo antigen antibody reaction include ADCC, toxin neutralization, inflammation, the complement system and neutralization of toxins amongst others; and in vivo antigen-antibody reaction is mainly aimed at protecting the host from the negative consequences of antigens or pathogenic microorganisms that penetrate the body.

Agglutination is simply defined as the visible clumping of particulate antigens (e.g. bacteria) by an antibody (otherwise known as agglutinins). Agglutination reaction can also occur with erythrocytes or red blood cells (RBCs); and this type of agglutination is generally known as haemagglutination reaction. Haemagglutination reaction is caused by haemagglutinins just as agglutinins and precipitins cause agglutination and precipitation reactions respectively. Agglutinins are antibodies that cause visible clumping of particulate antigens.

They are generally immunoglobulin molecules that produce such immunological reactions (i.e. agglutination) in which visible clumps are formed when antibodies react specifically with antigens. Such interaction result in the formation of visible clumps; and this principle of agglutination reaction is applied in microbiology laboratory (particularly in serology) to determine the presence of antigens or antibodies in patient’s clinical specimens (e.g. serum or plasma). The Widal test used for the laboratory diagnosis of Salmonella infection is based on the principle of agglutination test.

Precipitation reaction is the immunological interaction of a soluble antigen with a soluble immunoglobulin molecule (precipitin) to form an insoluble complex. Precipitation test is used to detect streptococcal group antigens. It is noteworthy that the reaction between an antibody and an antigen is highly specific in nature; and because of this specificity the notable interactions between immunoglobulin molecules and antigenic molecules have been exploited by medical scientists to determine the presence of either an antigen or antibody in clinical samples in the course of unraveling the causative agents of some infectious diseases. This is the basis for most serological tests carried out in the microbiology or serology laboratories of hospitals. Nevertheless, cross-reactivity can occur in some cases between an antigen and unrelated immunoglobulin molecule and vice-versa; and cross-reactivity usually occurs in scenarios where two antigenic molecules share corresponding epitopes, a biological phenomenon that stimulate an antibody to bind or interact with an unrelated antigenic molecule.

Cross-reactivity is the capacity of a given immunoglobulin molecule or T cell receptor (TCR) to react with multiple antigens that share common antigenic determinants or epitopes. A typical example of cross-reactivity is observed in ABO blood group antigens that possess polysaccharide antigens that contain similar oligosaccharide residues; and the cross-reactivity exhibited by ABO blood groups in humans has played significant roles in blood typing and even in transplantation or tissue/organ grafting. Agglutination reaction can also be employed in the identification of bacterial pathogens (e.g. Salmonella, Shigella, and Streptococcus species) in the microbiology laboratory.

In the typing of ABO blood group of humans (i.e. in haemagglutination reaction), a drop of blood from an individual is mixed with antiserum antigens (e.g. antiserum A or B) on a slide; and the formation of visible clumps by the test blood sample with any of the corresponding antiserum antigen confirms the individuals blood group. For example, if the person’s blood sample formed visible clumps with antiserum A, then the person’s blood group is blood group A. The specificity between antigens and antibodies as earlier said are used in a variety of quantitative or qualitative immunologic assays to diagnose the causative agent(s) of infectious diseases.        

Enzyme Linked Immunosorbent Assay (ELISA)

ELISA is the acronym for “enzyme linked immunosorbent assay”. It is an immunoassay or serological test that is used for the quantification or identification of specific antibodies or antigens in biological specimens (e.g. serum or blood). ELISA is mainly based on the principle of enzyme-substrate reaction. In ELISA, an enzyme (e.g. horseradish enzyme and beta-galactosidase enzyme) conjugated with an antibody reacts with a colourless substrate (i.e. a chromogenic substrate) to produce a coloured reaction or product. The intensity of the coloured product produced in the course of performing the ELISA experiment is measured with a spectrophotometer; and this is usually done by taking the optical density (OD) of the colour produced on the machine (i.e. the spectrophotometer or colorimeter).

ELISA is commonly used in clinical immunology/medicine as a screening test to detect HIV antibodies in patient’s serum; and ELISA technique can also be used to evaluate the presence of drugs, hormones, serum proteins antibiotics and pathogenic microorganisms in clinical samples. They can also be used to detect biomarkers for cancerous cells as well as antibodies for hepatitis B virus, parasites (e.g. Toxoplasma gondii) and rubella viruses amongst other viral agents. ELISA, an immunosorbent assay can also be used for the presumptive assay or quantification of proteins in biological materials. ELISA is a better alternative to radioimmunoassay (RIA) which basically makes use of a radioactively labeled substance or radioisotopes to detect antigens or antibodies in samples; and due to the health-risk of handling radioisotopes or radioactive materials because of possible human contamination, ELISA is most preferably used in most clinical and research laboratories when quantifying or detecting antigens or antibodies in specimens; and the test is performed in microtiter plates (an assay plate that comprises of 96 wells).

There are two procedures involved in the ELISA technique: the indirect ELISA (which detects the presence of antibodies in biological samples) and the sandwich ELISA (which detects the presence of antigens in biological samples). In indirect ELISA (which is commonly used in clinical medicine/immunology to assay for antibodies to HIV in patients serum), the sample to be analyzed (e.g. serum) which is suspected of containing the antibody of interest is added to an antigen-coated well (i.e. the antigen is immobilized on the well of the microtiter plate). The antigen is absorbed onto the well(s) of the microtiter plate during immobilization or incubation.

The antibody in the sample is allowed for some time to react with the antigen coated to the well(s) of the microtiter plate (Figure 11). (Note: The antibody in the sample being analyzed is known as the primary antibody). The microtiter plate is washed with a buffer/distilled water, and free antigen molecules (i.e. antigens that did not react or become attached to the antibodies in the samples) are washed in the process. An enzyme-conjugated antibody (known as the secondary antibody) which binds to the primary antibody (i.e. the antibody in the test sample) is added to the microtiter plate in order to evaluate or determine the presence of antibody bound to the antigen. (Specific antigen present in the test sample binds to the coated antigens on the wells of the microtiter plate).

Free or unbound secondary antibody is washed away. After washing, a substrate that reacts specifically with the enzyme is then added to the wells of the microtiter plate; and the enzyme reacts with the substrate to produce a coloured reaction or product. The absorbance of the coloured product is measured in a spectrophotometric device which gives a value that is used to infer the presence of the antibody of interest in the test sample. In sandwich ELISA otherwise known as the double antibody sandwich immunoassay, the wells of the microtiter plate are coated or immobilized with antibodies unlike in indirect ELISA where the wells are coated with antigens. The test sample containing the antigen is added to the wells of the microtiter plate containing the immobilized or attached antibody and allowed for some time to react.

The microtiter plate is washed to remove unbound or free antigen; and an antibody-enzyme conjugate that is specific for the bound antigen is then added to the wells of the microtiter plate. The microtiter plate is washed again to remove unbound or free antibody; and a substrate specific for the bound enzyme molecule is then added to the wells of the microtiter plate. After reacting with the substrate, the enzyme converts the substrate into a coloured product whose optical density (OD) is measured in a spectrophotometer. Sandwich ELISA is used in bacteriology for the laboratory diagnosis of some bacterial related infections or diseases included those caused by but are not limited to Salmonella species, Vibrio cholerae and Helicobacter pylori and food allergens amongst others.

Figure 11: Illustration of a microtiter plate.

IMMUNE SYSTEM CELLS AND THEIR FUNCTIONS

All the cells of the immune system inclusive of blood cells originate from the bone marrow during haematopoiesis from progenitor haematopoietic stem cells (Figure 12). Haematopoiesis is simply defined as the biological process involved in the formation of blood cells (i.e. white blood cells, red blood cells and platelets) and other cellular components of blood particularly the immune system cells such as the B and T cells in the bone marrow. During haematopoiesis, progenitor haematopoietic stem cells in the medullar of the bone or bone marrow proliferate into progenitor erythroid cells, myeloid cells and lymphoid cells. The erythroid cells proliferate into erythrocytes or oxygen-carrying red blood cells (RBCs) and reticulocytes (i.e. immature RBCs) while the myeloid cells proliferate into myelocytes or myelocyte cells (e.g. macrophages, granulocytes and megakaryocytes) which play critical roles in both adaptive and innate immune response. The lymphoid cells proliferate mainly into lymphocytes which include T cells and B cells or white blood cells (WBCs).

Figure 12: An overview of the development of cells that take part in immune response. Source: Online Textbook of Bacteriology by Dr. Kenneth Todar, Department of Bacteriology University of Wisconsin, USA.  

The immune system is a complex system or network that comprises of several cells and organs and other immunological products (e.g. antibodies) which work cooperatively to protect the body from infectious diseases/infections as well as from disease causing pathogenic microorganisms. Every living animal (inclusive of humans) are inundated and fashioned with these complex networks of cells and organs that help them to fight against the invasion and colonization of the body by pathogenic microorganisms. These immune system cells are found dispersed in every part of the body; and they mount immunological actions to counteract the negative consequences of invading antigens or pathogens.

The immune system is man’s natural mechanism of keeping infectious diseases at bay; and when the immune system is compromised (e.g. through malnutrition, stress and the presence of a debilitating disease like HIV/AIDS); the individual becomes exposed to plethora of infectious disease agents that invades the body, colonizes it and cause disease/infections. Thus the effectiveness of the immune system of the host is critical to the prevention of invasion and colonization of the body by pathogenic microorganisms. Lymphoid cells, lymphocytes, phagocytes, macrophages, dendritic cells, memory B cells, plasma cells, granulocytes, megakaryocytes, basophils, eosinophils, neutrophils, null cells and monocytes amongst others are some of the cells of the immune system; and these cells become mobilized to respond to any invading antigen or pathogen that threatens the normal physiological conditions of the host.

LYMPHOCYTES

Lymphocytes are mononuclear leukocytes that mediate both humoral or antibody-mediated immunity and cell-mediate immunity. B cells, T cells and natural killer (NK) cells are the main types of lymphocytes that make up the immune system (particularly the adaptive or specific immunity). The lymphocytes which can also be known as lymphoid cells (i.e. the B and T cells) are the major cells of the entire immune system; and they are mainly responsible for the key attributes of the adaptive or specific immunity such as discrimination between self and non-self molecules, specificity, diversity and immunological memory amongst other characteristics that the specific immune system is known for. While “B” lymphocytes derive their name from their site of maturation (i.e. the Bursa of Fabricius in birds and bone marrow in mammals/humans), the “T” cells also derive their own name from their site of maturation; and in this case the thymus.

B cells can recognize soluble or free antigens via their B cell receptors (BCRs) unlike T cells whose T cell receptors (TCRs) cannot recognize free antigens except for those antigens that have been bound, processed and presented by one of the MHC molecules (e.g. Class I MHC or Class II MHC molecules). As shall be seen later in this chapter, Class I MHC molecules present antigenic peptide molecules to cytotoxic T cells (TC cells) which expresses CD8 receptors/membrane molecules while Class II MHC molecules present antigenic peptide molecules to helper T cells (TH cells) which expresses CD4 membrane molecules. And aside the TC cells or CD8+ cells and TH cells or CD4+, other subpopulations of T lymphocytes with unique immunological functions also exist. B cells are lymphocytes whose most important function in the immune system is to secrete antibody-producing plasma cells following their specific interaction with antigens or foreign body. The primarily mature in the bone marrow; and B lymphocytes or cells also differentiate into memory B cells that remain quiescent and parade the body for a second appearance of similar antigen. B cells are the main mediators of humoral or antibody-mediated immunity (AMI).

T cells are lymphocytes that mature mainly in the thymus after production in the bone marrow; and they express T cell receptors (TCRs) for the recognition of antigenic peptide molecules displayed by major histocompatibility complex (MHC) molecules. They are the main mediators of cell-mediated immunity (CMI); and T cells or lymphocytes undergo differentiation after activation into effector cells especially T helper (CD4+) cells and T cytotoxic (CD8+) cells amongst others. CD8+ cells are mainly responsible for killing tumor cells, virus-infected cells, transplant cells, and parasites; and they also down-regulate the immune system as well as recognize peptide molecules displayed by Class I MHC molecules. CD4+ cells recognize and process peptide molecules displayed by Class II MHC molecules; and they secrete immunological molecules that help to stimulate other components of the immune system.

PHAGOCYTES

Phagocytes are generally known as bacteria-eating cells. They engulf microbial cells (particularly pathogenic bacteria) through a process known as phagocytosis, and this leads to the degradation of the engulfed bacteria. Macrophages, neutrophils and eosinophils are examples of phagocytic cells.

MACROPHAGES

Macrophages are multi-functional immune system cells that have a variety of immunological functions. For example, macrophages act as antigen presenting cells and they also process antigens and make them readily available for phagocytosis. Macrophages mediate phagocytosis and they also secrete cytokines as well as mediate antibody-dependent cell-mediated cytotoxicity (ADCC). Macrophages are found in various tissues of the body where they express a variety of immunological functions especially phagocytosis. Kupffer cells and histiocytes are macrophage-like cells found in the liver and connective tissues respectively. In the lungs and kidneys, alveolar macrophages and mesangial cells respectively are the macrophage-like cells that provide immunological functions. Microglial cells and osteoclasts are the macrophage-like cells that are found in the brain and bone respectively.    

DENDRITIC CELLS

Dendritic cells (DC) are cells of the immune system that mainly act as antigen presenting cells (APCs); and they comprises of langerhans cells found underneath the skin. They play critical roles in both innate and adaptive immune response. Dendritic cells are found in the blood, spleen, lymph nodes and thymus; and they are professional APCs like the macrophages. They basically process and present antigenic peptide molecules to T helper (CD4+) cells because they express receptors for Class II MHC molecules.

MEMORY B CELLS

Memory B cells are unique class of B cells that remain inert but viable in the blood circulation for a long period of time; and they are capable of rapid activation upon the encounter of a previously invading pathogen or antigen in the body. Memory B cells play critical role in secondary immune response; and each time the immune system is exposed to a particular antigen and produce effector B and T cells to counter the debilitating effect of the invading pathogen, the immune system also produces several memory B and T cells which police the entire blood circulation in search of similar antigens that invades the body in the future. These memory B and T cells mount a rapid secondary immunological response that activates and mobilizes other components of the immune system into action.

PLASMA CELLS

Plasma cells are antibody-secreting cells produced by immunocompetent B cells. They are short-lived in the blood circulation but plasma cells produces large amount of specific antibodies during their short lifespan. During their short lifespan, plasma cells secrete large amount of immunoglobulins that are specific for each of the antigenic determinant sites or epitopes of antigens that invades the body.

GRANULOCYTES

Granulocytes are a type of white blood cells (WBCs) or leukocytes that contain granules in their cytoplasm. Examples of granulocytes include basophils, neutrophils and eosinophils. Granulocytes are different from non-granular leukocytes (e.g. monocytes and lymphocytes) which do not contain granules in their cytoplasm. The granulocytes are produced in the bone marrow; and they specifically fight invading pathogens or antigens in the body.

MEGAKARYOCYTES

Megakaryocytes are multi-nucleated large cells that are produced in the bone marrow with the sole biological function of producing platelets or thrombocytes, blood-clotting factors.

BASOPHILS

Basophils are granulated WBCs or leukocytes; and they usually contain vasoactive substances such as histamine produced during allergic reactions in the body and heparin, an anticoagulant. They are mainly present in the blood circulation where they initiate inflammatory reactions following the body’s invasion by pathogens or antigens. Upon encountering an antigen, immunoglobulins (especially immunoglobulin E, IgE) through their FC region binds to or cross-links with the basophils; and this binding leads to the release of vasoactive substances or pro-inflammatory mediators (e.g. histamine) which results in hypersensitivity reaction in the body. Basophils stain blue when stained with basic dyes; and this aid in their identification in blood specimens in the laboratory.

EOSINOPHILS

Eosinophils are a type of granulocytes that can easily be stained with eosin, a red-crystalline dye. They have phagocytic properties; and unlike other types of granulocytes the eosinophils can phagocytose or engulf bacteria through the process of phagocytosis. Eosinophils also play critical roles in mounting immunological response during parasitic or nematode infection. Eosinophils stain red when stained with eosin.

NEUTROPHILS

Neutrophils are a type of granulocytes which exhibit both phagocytic and inflammatory immunological response. They stain neutral or pale pink when stained with Wright’s stain. Neutrophils are also known as polymorphonuclear leukocytes (PMNs); and they are generally known as bacteria-eating granulocytes. They are bacteriocidal in action.

NULL CELLS

Null cells are immature B cells that have not encountered an antigen. They can also be called naïve B cells or lymphocytes.  

MONOCYTES

Monocytes are nucleated leukocytes that exhibit phagocytic action following the invasion of antigens or pathogenic bacteria into the body.

MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) MOLECULES

Major histocompatibility complex (MHC) molecules also known as human leukocyte antigens (HLA) complex are a large set of cell surface protein molecules that are controlled by a collection of polymorphic genes known as the major histocompatibility complex (MHC) which are located on chromosome 6 in humans and on chromosome 17 in mice. The MHC is a set of gene family that encodes the production of MHC molecules; and they help the immune system to discriminate between self and non-self molecules. MHC molecules are the main inducers of cell-mediated immunity i.e. T cell immune response; and they also take part in humoral immune response.

The T cells unlike B lymphocytes cannot recognize antigens on their own except those antigens that have been presented by MHC molecules or are complexed with the MHC molecules. The major biological function of the MHC molecules in immune response is to signal to the T cells that a particular cell is infected; and this signal stimulate the cell-mediated immune response which is mainly dominated or controlled by T cells inclusive of T helper (TH) cells and T cytotoxic (TC) cells. Thus MHC molecules act as antigen presenting cells (APCs) since they help the T cells to recognize antigens.

MHC molecules are clinically significant because they can trigger T cell immune response in individuals receiving transplanted tissues or organs; and such reaction can lead to the rejection of the grafted organ or tissue since MHC molecules are present on the plasma membrane of virtually all human tissue cells and they are unique for each individual except for closely related individuals (e.g. identical twins). The MHC/HLA molecules serve as a unique biomarker of individual identity; and they are applied in a series of biomedical and medical applications in which HLA typing is used to determine paternity and also to determine HLA compatibility between donor and recipient hosts prior to transplantation or grafting.

It is worthy of note that the MHC (HLA) molecules where discovered from the notable works of George D. Snell and Peter A. Gorer in the early 1930s who both investigated tissue transplantation in inbred mice. The major histocompatibility (MHC) molecules or HLA plays critical role in immune response (inclusive of cell-mediated and humoral or antibody-mediated immune response). Most microbial infections or diseases are caused by intracellular microbes (e.g. viruses, protozoa and bacteria) that are well-known for living inside the cells of their host where antibodies and other components of the immune system cannot easily reach them.

Since the surfaces of host cells are lined with unique MHC/HLA molecules which help to distinguish self molecules from non-self molecules; the MHC molecules recognizes and binds to peptide molecules released by the intracellular parasites and thus displays these peptide molecules to the T cell component of the immune system which identifies and kill the invading pathogen. MHC molecules generally enable T cells to recognize and bind antigenic peptides that emanate from intracellular microorganisms; and they also play critical roles in immune recognition and discrimination. There are three main variants of the MHC/HLA molecules.

CLASS I MHC MOLECULES

Class I MHC molecules present peptide molecules from intracellular antigens (or pathogens) to T cytotoxic (TC) cells otherwise known as CD8+ cells. They are found on the surface of virtually all nucleated cells of the human body; and Class I MHC molecules are the most important HLA molecules recognized by recipient host cells during grafting or transplantation. Thus class I MHC molecules are able to stimulate immune response in recipient host receiving graft tissues or organs especially those with a different class I MHC molecules. This is the basis for HLA typing in clinical medicine in order to determine MHC compatibility between donor and recipient hosts prior to transplantation. Class I MHC molecules are principal in the lysis of virus infected cells; and their presentation of proteolytically degraded protein molecules from intracellular pathogens to TC cells (CD8+ cells) facilitate the initiation of cell-mediated immune response.

CLASS II MHC MOLECULES

Class II MHC molecules present peptide molecules from antigens or pathogens to T helper (TH) cells also known as CD4+ cells. They are mainly found on the surfaces of immunocompetent cells especially antigen presenting cells (e.g. macrophages, langerhans cells, B cells and dendritic cells). While class I MHC molecules present endogenously processed or synthesized peptide molecules to TC cells, class II MHC molecules only present exogenously processed peptide molecules internalized by APCs (e.g. macrophages). This implies that Class I MHC molecules only recognizes and present endogenously synthesized antigens or pathogens (e.g. viral particles and other intracellularly-placed pathogens such as Rickettsia and Chlamydia species) while Class II MHC molecules only recognizes and present the antigens of extracellular pathogenic microorganisms that have been engulfed by phagocytes. Such extracellular pathogenic microorganisms include bacteria and other microbial proteins that are usually degraded exogenously.    

CLASS III MHC MOLECULES

Class III MHC molecules are a group of free unrelated protein molecules which unlike class I and class II MHC molecules do not take part in the presentation of peptide molecules from antigens to T cells. They primarily take part in other immune system functions (e.g. the initiation of the complement system, heat shock proteins (HSPs) and inflammatory reactions). Class III MHC molecules do not share structural similarity with either class I or class II MHC molecules.

The ability of T helper cells or CD4+ cells to recognize only peptide molecules complexed with class II MHC molecules and for T cytotoxic cells or CD8+ cells to recognize only peptide molecules complexed with class I MHC molecules is because the MHC/HLA molecules exhibit major histocompatibility complex (MHC) restriction. MHC restriction is simply defined as the ability of the T cells (i.e. CD4+ and CD8+ cells) to specifically recognize antigenic peptide molecules presented by the right type of MHC molecules either Class I or Class II molecules. This class restriction exhibited by the MHC molecules is controlled by specific genes or binding sites found on the T cell receptors (TCRs); and these binding sites are unique for both CD4+ cells CD8+ cells thus the reason why they only recognize antigenic peptide molecules presented by Class II and Class I and Class I MHC molecules respectively.

TCRs generally recognize only antigens being presented by MHC molecules. And just like B cells differentiating and proliferating into antibody-secreting plasma cells and memory B cells after encountering an antigen; T cells also become activated, differentiate and proliferate into other effector T cells and memory T cells after encountering an antigen on its receptor (i.e. TCRs) through the assistance of the MHC molecules. T cells only bind to antigens complexed with any of the MHC molecules while B cells are capable of binding to soluble antigenic molecules without the help of the MHC molecules. And this is the basic difference between the cell-mediated immune response and the humoral or antibody-mediated immune response.

B CELL PRODUCTION

B cells are specialized type of lymphocytes that are responsible for the production of antibodies or immunoglobulins that act as effector molecules to stimulate the entire immune system against an invading antigen or pathogen. They are primarily responsible for the development of antibody mediated immunity (AMI) in the body; and B cells are produced in the bone marrow. In birds, B cells are produced in the Bursa of Fabricius, a lymphoid organ situated near the terminal end of the gut in birds or fowl (i.e. around the cloacae region). The removal of Bursa of Fabricius from chicken or birds (a medical procedure known as bursectomy) will impair antibody mediated immune response in the chicken because the affected birds will no longer secrete B cells which are biologically significant for the production of antibodies and memory B cells during the invasion of pathogens.

This also applies to mouse whose thymus is removed through thymectomy. Thymectomy is the medical procedure of removing the thymus through surgery; and this procedure exposes the animal to incessant attacks from pathogens especially intracellular parasites. Such mice would find it difficult to produce effector T lymphocytes because the thymus is the organ where T cells mature after their production in the bone marrow. Mice whose thymuses have been surgically removed will generally have an incapacitated cell-mediated immune response especially to intracellular pathogens including viruses which are known to reside within their host cells.

The bone marrow is the site where all the cells of the immune system (e.g. B and T lymphocytes) inclusive of blood cells (e.g. red blood cells, white blood cells and platelets) are initially derived from. They are derived from haematopoietic stem cells (HSCs) during haematopoiesis in the bone marrow. Stem cells are self-renewing cells found in the bone marrow and which undergo cell division to differentiate and proliferate into other important cells inclusive of the immune system cells and blood cells.

HSCs are pluripotent or multipotent in nature because they can differentiate into a wide variety of other cells (e.g. lymphocytes, erythrocytes). B cell maturation is a continuous process that occurs throughout life in the bone marrow of adults. However, the process of B cell maturation in neonates or fetus in utero (i.e. before birth) usually occurs in the fetal bone marrow, yolk sac or fetal liver; and after birth B cell maturation continues in the bone marrow. After production in the bone marrow, the B cells migrate to secondary lymphoid organs such as the lymph nodes where they differentiate into plasma cells that mediate or stimulate antibody production but only in the appearance of an antigen.

Apart from differentiating into plasma cells for antibody production; the B cells are also responsible for the presentation of antigens to T helper cells which facilitate cell-mediated immunity (CMI) in the host. Thus B cells can act as both antigen presenting cells (APCs) and synthesizing machinery for antibodies and memory cells. T cells as shall be seen later only recognize antigen molecules presented in association with major histocompatibility complex (MHC) molecules. Nevertheless B cells can recognize antigen molecules by itself and without the assistance of MHC molecules; and this is a major difference between B cells and T cells. T cell maturation occurs in the thymus; and its activation and differentiation occurs in the peripheral lymphoid tissue (e.g. spleen) after encountering an antigen presented by MHC molecule(s).

The activation of B cells is driven by the introduction of foreign bodies or antigens into the body; and once activated; B cells undergo proliferation and differentiation into numerous plasma cells for the production of abundant antibodies with unique specificity for antigens. During differentiation or proliferation, the B cells mainly develop into two types of cells viz: memory cells and plasma cells. While plasma cells are largely responsible for the production of antibodies and other effector molecules; the memory cells or memory B cells remain in the circulating blood or body fluids after the primary immune response to police the body and eliminate the invading antigen should the body become exposed to a previously exposed antigen or foreign body the second time (i.e. in the future); and this usually happens during a secondary immune response in which the immune system mounts a rapid attack against the invading pathogen. The stages involved in the production of immunocompetent B cells in the bone marrow are a complex set of biological process (Figure 13). However, these stages are highlighted and summarized in this section.

  • Maturation: At maturation stage, immunocompetent B cells are produced in the bone marrow by competent progenitor stem cells.
  • Activation: Once produced, the immunocompetent B cells become activated following their interaction with antigens or invasion of antigens.
  • Differentiation: After the interaction of the immunocompetent B cells with the antigen(s), the activated B lymphocytes differentiate into plasma and memory B cells.

Figure 13: Diagram showing the development of the cells of the immune system (B cells and T cells in particular). Source: Byer C.O, Shainberg L.W and Galliano G (1999). Byer/Shainberg/Galliano editions. Dimensions of Human Sexuality, 5th edition. The McGraw-Hill Companies Inc, USA.

The initial stage of B cell development in the bone marrow (known as antigen-independent stage) is carried out without the presence of any stimulating antigen or foreign body; and the B lymphocytes so produced are generally known as incompetent B cells because they have not encountered any antigen. Such B cells only circulate in the blood before being transported to the lymph nodes or spleen until a foreign body is encountered in the body. However, upon encountering an antigen, the B cells become activated and proliferate in order to differentiate into numerous plasma cells (for antibody production) and memory B cells for immunological memory should incase the antigen invades the body the second time in the future.

The proliferation of B cells at this stage into more B lymphocytes for the production of antibody-producing plasma cells and memory B cells is generally known as clonal expansion/selection. The term clonal selection and clonal expansion shall be used synonymously in this section. In clonal expansion/selection, a particular B or T cell undergo cell division and differentiate into clones of B and T cells respectively with unique antigenic specificity as that of the original B or T cell that they arise from.

Clonal expansion as shall be seen later in this section is the enlargement in the number of competent B cells or T cells with the same antigenic specificity as the progenitor (parent) B cell or T cell. Such B or T cells arise from the same clone of B or T lymphocytes. The expansion of B and T cells in this fashion is controlled by the clonal selection theory. This later stage of B cell development is known as the antigendependent stage because the activation and differentiation of B cells require a prior exposure to antigens. B cells have very short lifespan (about 8 weeks) and they die-off if they fail to encounter an antigen; and thus B lymphocytes circulate as incompetent B cells and compete amongst themselves for space in the secondary lymphoid organs (e.g. spleen) where they remain for the invasion of foreign bodies or antigens.

The naïve or incompetent B cells that die-off because they did not encounter antigen for activation and proliferation die through a biological process known as apoptosis (i.e. a programmed cell death. The formation of specific antibodies each and every time an antigen invades the host is mainly directed or mediated by the prior interaction of the foreign body with antibody-secreting B cells. It is noteworthy that the B lymphocytes are saddled with surface receptors e.g. membrane-bound IgD (mIgD) or membrane-bound IgM (mIgM) that react specifically with the invading antigen. After this interaction between the antigen and the immunologically responsive B cells, the B lymphocytes proliferate into clones of B cells that produce numerous antibody-secreting plasma cells that secrete immunoglobulins that are specific for the antigens.

A clone is a cluster of cells that arise from a single progenitor cell. This mechanism through which antibody-secreting plasma cells emanate from a collection of activated or immunologically responsive B lymphocytes is known as clonal selection; and clonal selection is largely responsible for the secretion of numerous antibodies with specificity for unique antigenic determinant sites or epitopes of antigens. Clonal selection as earlier stated is the immunological mechanism in which antigen-binding receptors of B cells and T cells stimulate the lymphocytes (i.e. B and T cells) to proliferate into a clone of lymphocytes that produce effector cells (e.g. antibodies, T helper cells and T cytotoxic cells) with the same antigenic specificity as their progenitor or parent cell.

It is naïve or immunocompetent B cells that become activated via antigen-antibody reaction, and then differentiate or proliferate into numerous antibody-secreting plasma cells and memory B cells.   The number of B or T cells produced during clonal selection/expansion is amplified; and the process ensures that sufficient amount of memory lymphocytes (B and T cells inclusive) with unique antigenic specificity are always available to assuage the activities of invading pathogens or antigens in the body. Aside the secreted antibodies, the immunologically responsive B cells also proliferate into memory B cells which remain dormant but viable in the blood circulation until a later time when similar antigen invades the host’s body again; and memory B cells explain the reason for the rapid response of antibody production in secondary immune response.

Effector cells of the immune system are immunologically responsive cells that directly encounter antigens and facilitate their neutralization and possible elimination from the host. Examples of effector cells include plasma cells and T cells. Plasma cells are the effector cells of the humoral or antibody-mediated immunity mainly responsible for antibody secretion while the effector cells of the T cells are T helper cells (CD4+) and T cytotoxic cells (CD8+). Self reactive T lymphocytes (i.e. T cells that attacked host cells instead of antigens) are eliminated from the thymus through the process of clonal deletion. In summary, the proliferation of B cells and T cells into effector cells is largely governed by the clonal selection theory; and this theory postulates that clones of effector B and T cells arise from a single parent cell that is stimulated via antigen-binding to reproduce similar lymphocytes with unique immunologic specificity and memory.

The clonal selection theory postulates that all the antibody molecules on a single B cell (i.e. antibodies that originated from a single progenitor or parent B cell) have the same antigenic specificity as their parent B cell; and this also apply to the differentiation of naïve T cells into immunocompetent T cells with antigenic specificity as that of their parent T cell. It is a biological theory that explains the unique specificity of antibody formation; and the clonal selection theory is generally based on the certainty that each lymphocyte is programmed to produce a particular type of immunoglobulin that is eventually selected by prior contact with an antigen or pathogen.

All the B and T cells in the expanded parent clones are specific for the epitopes of the original pathogen or antigen that activated their production and selection/expansion into effector cells (i.e. memory B cells, memory T cells and immunocompetent B and T cells). The clonal expansion of T cells occurs after the T cells have encountered an antigen presented by any of the MHC molecules; and the T cells (e.g. CD4+ helper T cells) secrete cytokines which go on to stimulate an antigen-activated B cell to proliferate and differentiate into antibody-secreting plasma cells that eventually produce specific antibodies that bind and facilitates the degradation and removal of the invading pathogen or antigen. This explains how the B cells and T cells cooperatively neutralize the activities of antigens in the body.

ORGANS OF THE IMMUNE SYSTEM AND THEIR FUNCTION

The organs of the immune system are generally divided into two major groups which are the primary lymphoid organs and the secondary lymphoid organs. The bone marrow, spleen, thymus, Peyer’s patches, liver, the tonsils, the lymph nodes and several mucosal-associated lymphoid tissues (MALT) are some of the major organs or tissues that makeup the immune system of humans and other mammals (Figure 14). These organs play unique and various critical roles in the development of immune response against an invading pathogen or antigen; and they are specialized types of organs or tissues that makes the immunological system outstanding from other systems of the body. Primary lymphoid organs are the central organs of the immune system; and it is in these organs that the maturation of the immune system cells (particularly the B and T lymphocytes) primarily occurs.

The primary lymphoid organs primarily serve as sources of lymphocytes for other components of the immune system. Bone marrow and thymus are the two main organs that makeup the primary lymphoid organs. In the primary lymphoid organs, haematopoiesis and the generation of lymphocytes occurs; and these organs are the central sites where lymphocytes (i.e. B and T cells) develop the characteristic features that typify the adaptive immune system such as discrimination between self and non-self molecules. It is also at the primary lymphoid organs that the lymphocytes develop their first ability to specifically recognize antigens or foreign bodies that invades the body. The lymphocytes that are produced in the primary lymphoid organs are initially immature or immunocompetent cells until they become committed to a particular antigen within these organs and then transforms into immunocompetent cells with unique antigenic specificity.

Figure 14: Schematic illustration of the organs of the human immune system.

Tonsils are found at the base of the tongue and at the side of the back of the mouth where they defend against pathogens that enter the body through the mouth; the adenoids are located in the nasopharyngeal roof where they defend against antigens that enter the body via the nasal passage or opening.

The thymus is the organ where T cell maturation occurs after its production in the bone marrow and it is located above the heart;.

The bone marrow is where B cell development and maturation as well as the development of other haematopoietic stem cells such as RBCs and WBCs occur and it is usually located in long bones of the body.

Lymph nodes are found all over the body especially along the junctions of the lymphatic vessels where they provide a microenvironment for the trapping, processing and presentation of antigens (trapped from regional tissue spaces in the body) to other components of the immune system.

The spleen is located at the left abdominal cavity and below the pancreas where they filter the blood to trap blood-borne pathogens.

The peyers patches are located within the gastrointestinal tract (GIT) and even in the appendix where they provide immunological response.

The lymphatic vessels is primarily responsible for the distribution of antigens that entered the tissues to other lymphoid organs for specific immunological response.

While the development and maturation of B cells occurs in the bone marrow, the development and maturation of T cells occurs in the thymus. A lymphocyte becomes immunocompetent only when it has matured within a primary lymphoid organ (e.g. thymus and bone marrow); and immune system cells that are immunocompetent are B and T cells that has the ability to mount an immunological response against a particular antigen or pathogen in the body of an animal or mammal. Immune system cells become immunocompetent after their interaction with antigens.

The bone marrow is a yellowish-soft tissue found in all the major bones of the body (especially the long bones); and it is the primary site where all the cells of the immune system are derived from during haematopoiesis. Aside the B cells which develop and mature in the bone marrow, other cells of the immune system that develop from the bone marrow during haematopoiesis from HSCs include the progenitor T cells, mast cells, dendritic cells, NK cells, erythrocytes, platelets and granulocytes.

In neonates or foetus in utero, B cell development initially occurs in the fetal liver, fetal bone marrow or yolk sac before continuation in the adult bone marrow. In birds, B cell development and maturation occurs in the Bursa of Fabricius. After their initial development and maturation in the bone marrow, the lymphocytes leave the bone marrow to continue their maturation in the peripheral lymphoid organs (i.e. the secondary lymphoid organs) as shall be seen later in this section.

It is noteworthy that the B cells produced during haematopoiesis of HSCs in the bone marrow are B lymphocytes that only attack non-self molecules (i.e. antigens or pathogens). However, some B cells with specificity for self molecules may be produced in some circumstances; and in such scenarios the clonal selection process that frequently occur in the bone marrow during haematopoiesis always ensures that B lymphocytes with self-reactive immunoglobulin receptors (i.e. B cells that attack host cells instead of antigens) are immediately eliminated. This also applies during T cell maturation in the thymus; and the process ensures that only T cells that recognize non-self molecules are produced and propagated.

During these processes of clonal selection of T and B cells with specificity for only non-self molecules, the lymphocytes are educated to attack only antigens and not self molecules; and only the population of lymphocytes with antigenic specificity for foreign bodies are allowed to proliferate while those that evoke detrimental autoimmune response in the host are immediately eliminated. The bone marrow as earlier stated is the site where all the cells of the immune system originate from; and it is also responsible for the production of other important cells of the body such as platelets and RBCs.

The thymus is a bilobed endocrine gland or organ that is located above the heart; and it is the main site for the maturation of immature or progenitor T cells that emanates from the bone marrow. It is made up of two main compartments, the medulla (the inner part) and cortex (the outer part) which both contain thymocytes. Thymocytes are immature T cells of the thymus from which mature or immunocompetent T lymphocytes develop from. The major biological function of the thymus is to ensure the continuous production and selection of immunocompetent T lymphocytes that recognize and processes antigenic peptide molecules presented by or complexed with the MHC molecules.

Secondary lymphoid organs are specialized organs or tissues where lymphocytes continue their maturation after their first production in the primary lymphoid organs (e.g. bone marrow and thymus). The secondary lymphoid organs are different from the primary lymphoid organs because it is within the secondary lymphoid organs that the lymphocytes (i.e. the B and T cells) proliferate and differentiate into effector cells that actually neutralize invading pathogens or antigens. For example, the B cells proliferate or differentiate into antibody-secreting plasma cells and memory B cells (which are the effector cells of the antibody-mediated immunity) while the T cells differentiate into effector T cells and memory T cells which are mainly responsible for cell-mediated immunity (CMI). Secondary lymphoid organs or tissues are mainly located along the vessels of the lymphatic system; and secondary lymphoid organs provide environment where effector lymphocytes interact with antigens or foreign bodies.

Lymphatic system is a series of vessels that are primarily responsible for the transportation of lymph fluids (i.e. pale-like biological liquids containing WBCs amongst other substances) from the tissues via the lymph nodes and into the entire circulation of blood in the body. The interaction of the lymphocytes with antigens within the secondary lymphoid organs leads to the differentiation of clones of lymphocytes into effector cells with unique antigenic specificity for particular antigens. The lymph nodes, spleen, and several mucosal-associate lymphoid tissues (MALT) are examples of the secondary lymphoid organs or tissues. The secondary lymphoid organs are also sites where antigens are presented to other cells of the immune system for immunological response. The lymph nodes and the spleen are the most important secondary lymphoid organs because of the fundamental roles they play in the processing and presentation of antigens to effector lymphocytes and other cells of the immune system.

Lymph nodes are encapsulated bean-shaped lymphoid structures or tissues found throughout the body system (Figure 15); and they comprises of lymphocytes (i.e. B and T cells), macrophages and dendritic cells amongst other key immune system cells that form a mesh of immunological system. The lymph nodes generally serve as sites for the filtration of antigens from the lymph fluid or lymph prior to their presentation to immunocompetent lymphocytes and other immune system cells located within the lymph node; and the lymph nodes are richly supplied by lymphatic vessels that ensure continuous supply of lymph fluids. Lymph nodes also drain fluids from other tissues of the body especially the regional or surrounding tissue spaces aside those supplied by the lymph.

Figure 15: Schematic illustration of the lymph node. The germinal center of secondary follicle is mostly inundated with B lymphocytes.

The lymph nodes are the sites where immune responses are mounted to antigenic molecules in lymph or lymph fluids; and they are mainly located at the junctions of the lymphatic vessels all over the body. The interdigitating or cellular networks formed by phagocytic cells and dendritic cells within the lymph nodes traps particulate antigens or pathogenic microorganisms that enters the lymph nodes through the lymph; and lymphocytes swim into action to mount immunological response against the trapped antigens. The cortex, paracortex and the medulla are the three main morphological sections or parts of the lymph node. The cortex is the outermost part of the lymph nodes, and it is rich in macrophages, lymphocytes (particularly B cells) and dendritic cells. The paracortex underlie the cortex, and it is rich in T lymphocytes. Dendritic cells can also be found in the paracortex. The medulla is the innermost part of the lymph node, and it is sparingly populated with antibody-secreting plasma cells and other cells with lymphoid origin.

They trap foreign bodies or antigens in the process prior to returning the lymph or lymph fluids they contain or are made of to the systemic circulation. Lymphocytes in the lymph nodes become activated, proliferate and differentiate into numerous effector molecules following the invasion of antigens. After the interaction of the antigens with the macrophages or dendritic cells situated within the lymph nodes, the antigen is processed and presented to immunocompetent B and T cells for the instigation of appropriate immunological response against the pathogen. Lymph fluids containing antigens or pathogens are brought into the lymph node via the afferent lymph duct or lymphatics of the lymph node while the efferent lymphatics is the passageway through which immunoglobulins and lymphocytes leaves the lymph nodes to the bloodstream (Figure 15). The spleen is an oval-shaped structure that is located in the upper left abdominal cavity. It is the site where antigens or pathogens are trapped from the blood circulation, and outworn RBCs are also destroyed within the spleen.

The spleen unlike the lymph nodes primarily filters the blood that passes via it and it trap antigens in the process and present them to the lymphocytes in the peripheral lymphoid tissues. It also traps antigens from other local tissues aside those carried through the blood that passes through it. Lymphocytes and blood-borne antigens are carried into the spleen via blood vessels (e.g. the spleen arteries); and the foreign bodies are presented to the lymphocytes in the process for appropriate immunological response. Note: While the lymph nodes only trap antigens from the lymph the spleen traps antigens in the blood.

The mucosal-associated lymphoid tissues (MALT) are secondary lymphoid organs or tissues that line the mucous membranes of the GIT, urogenital tract, respiratory tract and other mucous membrane surfaces that are found in the body. The primary function of MALT is to defend the mucous membranes surfaces of the body against invasion and colonization by pathogenic microorganisms or antigens. MALT produces numerous amounts of plasma cells that produce antibodies with unique antigenic specificity. Typical examples of tissues that makeup the MALT includes Peyer’s patch (which lines the GIT) and tonsils (which are found at the base of the tongue) which express several immunological functions at the sites they are located.

SIGNIFICANCE OF MONOCLONAL AND POLYCLONAL ANTIBODIES

The two basic forms in which antibodies can be produced in purified forms in the laboratory are as monoclonal antibodies and polyclonal antibodies. Monoclonal antibodies are homogenous antibodies produced in the laboratory by a single clone of cells (known as hybridoma cells), and all of which exhibit the same antigenic specificity. Hybridomas are cells capable of continues proliferation, and such cells possess the immortal growth features of cancerous plasma cells otherwise known as myeloma cells. Monoclonal antibodies are produced by a single clone of hybridized B cells; and they are uniform in their structure and specificity because they all arose from the same line of B lymphocytes.

Since an individual B lymphocyte is known to produce a specific immunoglobulin molecule in vivo, clones of such B cells can as well secrete homogenous antibodies with the same antigenic properties. This is the rationale behind the production of monoclonal antibodies for clinical and other laboratory or industrial applications. Nevertheless, the B cells recovered from an excised lymph node cells or spleen has very limited or short lifespan and as such these B lymphocytes from immunized laboratory animals cannot be cultured in vitro to produce or secrete sufficient amount of monoclonal antibodies for clinical usage.

Since the advent of hybridoma technology, monoclonal antibodies can now be synthesized in sufficient large amounts, and the processes involved shall be enumerated in this section. In the hybridoma technology of producing monoclonal antibodies, isolated B cells from immunized mouse are fused with myeloma cells from the same animal species in in vitro cell culture suspensions; and this produces hybrid B cell lines that are immortal in the production of homogenous antibodies (i.e. monoclonal antibodies) with uniform antigenic specificity. The steps involved in the production of monoclonal antibodies are succinctly enumerated as follows:

  • Inject or immunize a mouse with the antigen of interest.
  • Allow immunized mouse for a couple of weeks to generate antigen-specific B lymphocytes for the secretion of antibodies in the mouse.
  • Euthanize and remove the spleen or lymph nodes of the mouse.
  • Extract the B cells from the homogenized spleen or lymph nodes.
  • Fuse the extracted B lymphocytes with myeloma cells in in vitro cell culture techniques in suspensions. Addition of polyethylene glycol facilitates the fusion of the two cells.
  • Add hypoxanthine, aminopterin and thymidine (HAT) to generate hybridoma cell lines.
  • Hybridoma cells are isolated, separated and purified for monoclonal antibody production. Basically, the resulting clones of cells are screened in ELISA techniques for example in order to elucidate the production of specific antibody to the antigen of interest.

Monoclonal antibodies have a wide variety of applications, and these shall be highlighted in this section.

  • Monoclonal antibodies are used for the analysis or typing of viral pathogens and other microbial cells from clinical specimens.
  • Monoclonal antibodies are used as reagents for blood grouping experiments.
  • Monoclonal antibodies are veritable tools in the production of therapeutic reagents.
  • Monoclonal antibodies coupled with radioactively labeled probes can be used for the in vivo detection of cancerous antigens.
  • Monoclonal antibodies are used in the production of rapid diagnostic test kits and reagents.

Polyclonal antibodies are heterogeneous antibodies produced from different clones of B cells. While monoclonal antibodies only recognize antigens with single epitopes, polyclonal antibodies recognize antigenic molecules with multiple antigenic determinant sites or epitopes. Polyclonal antibodies are usually generated following the invasion of the body by antigens with multiple epitopes. Such antigens with multiple antigenic determinant sites stimulate the production of several B cell clones that proliferate and secrete heterogeneous antibodies specific for particular epitopes. The reproducibility of polyclonal antibodies the same way monoclonal antibodies are generated is not feasible and this is due in part to the multiple antigenic determinant sites or epitopes associated with polyclonal antibodies. Polyclonal antibodies are mainly applied in the development of immunoassays for laboratory diagnosis of infectious diseases.

INNATE IMMUNITY

Innate immunity is the body’s natural inborn resistance to infection and it is quick in responding to invasive microbes. It is a component of the immune system that is an inherited protective mechanism, and which protect the body of an animals from many kinds of pathogens. Innate immunity is a nonspecific immune response to antigens, and it is the first line of defense that is not acquired through prior contact with an infectious disease or pathogen. This form of immunity is not directed at any specific organism or antigen, instead they are intended for all pathogens or foreign substances to which the body is exposed. The response of the innate immune system to pathogen is static (i.e. it does not improve with repeated exposure to antigens and there is no immunological memory on subsequent exposures), and subsequent exposure of the body to antigens does not change or increase the level of response of this type of immunity. Innate immunity is comprised of four (4) types of defensive barriers which help to keep away pathogenic microorganisms from the host body, and these are:

  1. Anatomic barriers such as the skin, epithelium, gastrointestinal tract (GIT), mucous membrane, mammary gland (breast), respiratory tract, and urinogenital tract.
  2. Physiologic barriers such as body temperature, pH level, chemical mediators (e.g. cytokines, lysozyme, antimicrobial peptides, interleukins, surfactants, lipids, enzymes, interferon, and complements).
  3. Phagocytic barriers which include special forms of the white blood cells (e.g. monocytes, neutrophils, and eosinophils) which internalize/engulf and digest or breakdown foreign substances including whole microorganisms that enter the body.
  4. An inflammatory barrier which occurs following tissue damage and it is usually characterized by redness, swelling (oedema), pain and heat. In this type of innate immunity, vascular fluids containing protective serum proteins and phagocytic cells are released to the site of infection to stop the spread of the infection.
  5. Intact normal microflora: Normal microflora are populations of microorganisms (normally bacteria and fungi) which inhabit the internal and external body of healthy individuals. They are either resident or transient on the body, and they play a major role in protecting the body from foreign substances by antagonizing the colonization of body surfaces by pathogenic microorganisms. Normal microflora play a major role of microbial antagonism, and when they are not present or disturbed, pathogens colonize, proliferate and cause disease. The human body is in constant association with microorganisms (both beneficial and harmful organisms), and it is the job of microflora to ensure that harmful organisms do not colonize it. Normal microflora also outcompete microbes for essential nutrients in the body.

In healthy individuals, most of the microorganisms encountered by the body of a human host are readily cleared and eliminated by the mechanisms of the innate (nonspecific) immune system even before they activate the adaptive immune response to the invading foreign substance. Innate immune response is very vital as the first line of defense against an infection because it provide the very primary defense during the critical stage of infection (i.e. at the onset of host exposure to an antigen) since the adaptive (specific) immune response takes some time to come up.

However, there are some hosts factors that can affect and cause variations in the response of innate immunity based on individual differences such as: age, sex, nutrition, host genetic determinants, stress, hormonal activity, and fatigue. Some diseases or infections that occur in humans are sex-specific. Malnutrition also predisposes the body to infection because undernourishment affects the immune system and weakens its ability to protect the body from foreign substances or immunogens. Newborns and the elderly are more prone to some infections due to the low level of their immune system which is either still developing (as in infants) or is in a declining stage (as in old people). Stress, fatigue, hormonal imbalance and the genetic makeup of individuals are other predisposing factors that may affect the level of response of the immune system of a human host due to differences in their physiological and environmental conditions.

MECHANISMS OF INNATE IMMUNITY

  • The skin: The skin is an intact, tough and impermeable barrier that covers the human body and protects it from microbial invasion. When the integrity of the intact skin is breached or cut open as a result of burns, needle prick, insect bite, abrasions or wounds, the skin becomes access way via which infectious agents enters the body. The skin has a low pH, and in addition to this, it also secretes some antimicrobial substances such as fatty acids and lactic acids in sweats which help to keep pathogenic microorganisms at bay. The dry nature of the skin coupled with the dense population of resident microflora on it help to keep foreign substances and microbes in check or under control. The mechanical barrier of the intact skin, the presence of lysozyme (enzyme that dissolves the cell walls of some bacterial) and the acidic nature (pH 3-5) of the skin help to impede the growth of microbes.
  • Mucous membrane: Mucous membranes are largely found in the nose, mouth, gastrointestinal tract (GIT), cervicovaginal tract, urinogenital tract and the respiratory tract where their main function is to trap pathogenic microorganisms and other foreign substances that tries to enter the body via any of these routes. Mucous is a biological secretion produced by membranes that line the inner surfaces of the body. They contain substances or compounds that are either bactericidal or bacteriostatic to invasive microbes. Lysozyme is a typical example of an antimicrobial compound secreted in mucus. Microbial attachment to epithelial surfaces of the body is usually the first stage towards the development of an infection. Thus, mucous secretions containing secretory antibodies help to prevent this microbial attachment from taking place. Invasive microbes trapped by mucous secretions of the body are removed by reflex actions such as sneezing, coughing and cilliary movements of the respiratory tract which drives microbes upwards and outwards. The flushing mechanisms of the urine, tears and saliva also help to trap and remove microbes from the urinogenital tract, eyes and the mouth respectively. In the vagina of adult females, the mucous from the vaginal wall contains normal flora (e.g. lactobacillus) which maintains acidic pH that prevent the colonization of pathogenic microorganisms in that part of the body. Also in the GIT, a low pH and anaerobic environment is maintained by the resident normal flora in order to inhibit and prevent colonization by microbes.
  • Inflammation: Inflammation is the general response of the body to microbial invasion or tissue injury (e.g. wound). It is a complex series of immunological response (comprising of both the humoral and cellular immunity) that usually involves the accumulation of immune system cells and fluids around damaged body tissues or injuries. Microbial invasion elicits inflammatory response by causing the leakage of phagocytes and vascular fluids that contain serum proteins with antibacterial activity into the affected body area. This response helps to defend the body against infection and possible tissue damage. Cytokines and interferons (IFNs) are some of the chemical mediators released during an inflammatory response. The inflammatory response that occurs during microbial invasion or tissue damage is strong enough to prevent the spread of microbes including viruses, bacteria, and fungi from their portal of entry to other body sites.
  • Phagocytic action: Phagocytic action is mediated by special types of cells known as phagocytes which swim into action following microbial invasion of the body. Phagocytes are recruited in large numbers and they travel to the site of inflammation and move around in the blood circulation to eat up or engulf microbial cells and antigens. The process by which phagocytes ingest extracellular particulate materials (i.e. microbes and foreign bodies) is known as phagocytosis. Phagocytosis is an important innate defense mechanism that is carried out by a variety of cells collectively called phagocytes. The main functions of these cells are to ingest, kill and digest whole microorganisms.
  • Cytokines: Cytokines are low-molecular weight molecules of the innate immune system which are released by leukocytes to help in the regulation of inflammatory response during microbial invasion. They are chemical mediators which play specific augmenting and regulatory roles in innate defense mechanism. Some examples of cytokines include interferons (IFNs), interleukins (ILs), and tumour necrosis factor (TNF). Cytokines also provide innate immunity against viruses and tumour cells in host cells.
  • Complements: Complements are a series of serum proteins that play significant role in innate defense mechanism by catalyzing the killing of bacterial cells and also facilitating the migration of white blood cells to sites of inflammation or infection in the body. This group of independent serum proteins which works as a system and in cascade is generally known as “complements” because they balance or assist the antibody response of the immune system in the face of microbial invasion. Complements are heat-labile glycoproteins found in blood plasma and serum, and they help bacterial opsonization by antibodies. Specifically, complements consist of nine (9) numbered components (i.e. C1 – C9) in humans; and these are enzymatically activated through a series of pathways that make up the complement system. These numbered components of the complement are the individual proteins that make up the complements, and as earlier said, each is assigned a number from 1 – 9 to read C1, C2, C3, C4 and so on till C9. Each of these individual proteins of the complement system plays specific roles in both the innate and adaptive immune response. Complements are usually designated with the letter ‘C’, and this is usually followed by a number which designates the type. They are also present in tissue fluids and plasma; and complements are also known to help improve or augment the mechanism of the adaptive immune response since they are activated via antigen-antibody reaction. Complements play significant role in defending the host against infection and pathogen invasion in that they help to mediate phagocytosis, bacterial lysis, opsonization and inflammation.
  • Mammary glands: The mammary gland (i.e. the breast in females) is an anatomical structure that provides a non-immunologically surface protective mechanism in humans. The flushing action of the milk that emanates from the breast helps to prevent the invasion of foreign bodies and microbes. Also, the milk contains some antimicrobial agents or molecules (e.g. lysozymes) that lyse bacterial cells. In addition, the first milk from the mammary gland (known as colostrum) helps to initiate some level of immunity in newborns.

CELLS OF INNATE IMMUNITY

Innate immunity is found in nearly all forms of life, and exposure to foreign substances leads to immediate maximal response mediated by cells of the innate immunity. The cellular components of the innate immunity include natural killer (NK) cells, phagocytic cells, and cells of the reticuloendothelial system.

1. NATURAL KILLER (NK) CELLS

Natural killer (NK) cells are large granulated lymphocytic cells of the immune system which are found in the blood circulation, and which are primarily saddled with the responsibility of identifying and killing virally-infected cells. They are large lymphoid granulated cells that identify and destroy certain abnormal cells in the body (e.g. tumour cells and intracellular pathogens); and NK cells release large killing chemicals such as granyzymes and perforins from its granules. Both granyzymes and perforins help to induce apoptosis (i.e. programmed cell death) in virally-infected and cancer cells. NK cells are non-B and non-T cells, and they are largely present in non-immunized individuals where they produce a MHC-dependent cytolytic or killing activity against cancer cells and viral infected cells. Thus, NK cells do not require any prior antigen activation before they destroy virally-infected cells, and they do also do not exhibit immune memory. Once activated, NK cells produce a wide variety of cytokines such as interleukins, interferons and tumour necrosis factor that help to regulate inflammation.

NK cells are circulating defensive cells that eliminate invading pathogens and antigens from the body. Generally, NK cells possess two distinctive cell surface receptors viz: killer activation receptor and killer inhibition receptor which activate and inhibit the release of cytokines respectively. These cell surface receptors on NK cells help to recognize glycoproteins on the surface of virally-infected cells and cancer cells (for killer activation receptor). NK cells interact with normal body cells as they are circulated in the body. Thus they recognize normal body cells and their MHC class I molecules which help to deactivate the killing mechanisms of granyzymes and perforins when NK cells bind to the MHC class I receptors on normal body cells. In the absence of these MHC class I molecules on normal body cells, the killer activation receptor of the NK cells becomes activated to destroy the virally-infected and cancer cells.

2. PHAGOCYTES

Phagocytes are cells of the immune system that recognize, digest and destroy microorganisms and other foreign particles that enter the body. They principally carry out the process of phagocytosis i.e. the recognition, ingestion, and degradation of microbes and particulate matter by certain types of white blood cells. They are circulating defensive cells like the NK cells which eliminate microbes from the body. Macrophages, neutrophils, eosinophils and other related cells are some examples of phagocytic cells. Neutrophils are a type of white blood cell (WBC) with an irregular nucleus, and which can attack and kill invading pathogenic bacteria. Eosinophils are polymorphonuclear (PMN) granulocytes with phagocytic action and cell-surface-receptor-sites for the attachment of antibodies especially IgE and IgG. Neutrophils, basophils and eosinophils are different types of WBCs or leukocytes found in granulocytes (i.e. leukocytes that contain granules). Phagocytes which are often referred to as polymorphonuclear leukocytes engulf whole microorganisms and their fragments from their immediate extracellular environment, and they increase in number during bacterial infection or invasion. Phagocytic cells travel to sites of inflammation in the body by means of chemotaxis and they attach to the invading microorganisms through a non-specific cell surface receptor to initiate the mechanism for microbial destruction. The prime function of phagocytes is “to eat bacteria”.      

3. RETICULOENDOTHELIAL SYSTEM (RES)

The reticuloendothelial system (RES) is a collection of macrophages (or mononuclear phagocytes) that are found in specific organs of the body where they help to get rid of antigens and other microbial cells from the body’s circulation of blood. Some examples of the organs that make up the RES include the bone marrow, liver, lungs and the spleen. RES act as microbial filters in that they help to sieve or sort out microbial cells from the blood circulation so that they do not spread and cause bacterial sepsis.

4. MAST CELLS

Mast cells are granulated tissue cells derived from the bone-marrow and which initiate rapid inflammatory response. They resemble basophils, and mast cells release inflammatory mediators which activate vasodilation and the migration of phagocytes to sites of inflammation. Basophiles are a type of white blood cell which has granules in its cytoplasm and contains histamine and heparin (an anticoagulant). Mast cells respond to certain external and internal stimuli in the body by promptly secreting a variety of pro-inflammatory mediators or vasoactive products such as histamine and prostaglandins which play significant role in hypersensitivity and inflammatory reactions.

Mast cells only occur in lymphatic systems and other blood vessels; and they are not circulating cells like the other cells of the immune system (e.g. basophils). The similarity between mast cells and basophils lies in the fact that both contain dense granules in their cytoplasm. Mast cells have receptors for the Fc (crystallizable fragment) region of immunoglobulin E (IgE); and the binding of mast cells to this region of IgE results in degranulation of the cell. Degranulation of mast cells is the release of granules or vasoactive products (e.g. leukotrienes, heparin, and histamines) inherent in the mast cells following the binding of the Fc region of IgE to receptors on the surfaces of mast cells after a bacterial invasion of the body.     

5. DENDRITIC CELLS

Dendritic cells are special type of leukocytes found in the skin, thymus, spleen and lymph nodes, and which primarily act as professional antigen presenting cells. They are motile and non-phagocytic adherent cells that contain numerous mitochondria and nucleus. Dendritic cells form an important association between innate and adaptive immune response due to their antigen presenting ability. They resemble the dendrites of the neurons that make up the central nervous system (CNS) because they are surrounded with long membranous extensions as those of the former; and dendritic cells specifically express MHC class II molecules and they present antigens to only T-helper cells. Examples of dendritic cells include langerhans cells found in the skin and interdigitating cells found in the lymph nodes.

ADAPTIVE IMMUNITY

Adaptive immunity which can also be called acquired resistance to infection is an antigen-specific type of immunity that is an acquired ability of a host immune system to recognize and destroy specific microbes, their products and other foreign bodies that invade the body. It is a specific type of immune response that is elicited based on the previous exposure of a host to a particular pathogenic microorganism or antigen. The adaptive (acquired) immunity response is usually two-fold viz: humoral response (antibody mediated) and cellular response (cell mediated); and they develop slowly but their effect in the body as it regards to immunity against infection is long lasting. Humoral immune response and cell-mediated immunity are the two main divisions that comprise the adaptive immune response.

Generally, the prime functions of the adaptive immune response is to recognize specific antigens or microbes, develop a reaction or feedback to the epitope of the antigen and finally, to establish an immunological memory of the encountered pathogen in order to stimulate a robust and rapid response against it in case it attacks the second time. It is the second line of defense against infection, and it normally swims into action when the mechanism of the innate immunity fails.

The innate immune response on its own cannot sufficiently protect an individual against invading microbes and foreign bodies; thus it is the responsibility of the acquired immunity to enhance the general response of the immune system to microbial invasion particularly when the natural (innate) immunity is inundated by the antigenic attack. Adaptive immunity also include the activation of some less specific components of the immune system such as the complements, macrophages, NK cells and complements which help to improve the general immunological response to an invading pathogen. B cells and T cells are the main components of the cells of the adaptive immune response. It is noteworthy that microbes in their ingenuity can mutate overtime to evade the attack of the innate immunity and its associated components.

However, the host body has evolved line of attack (i.e. the adaptive immunity) which it uses to dislodge specific immunogens that invade the body no matter how transformed they may look to the innate immune mechanism. Following the entry of microbes and foreign bodies into the body, the innate immunity and its associated components interacts with the antigens and they are further presented as peptide molecules on antigen-presenting cells (APCs) which are complexed by MHC molecules and finally presented to T lymphocytes for destruction. The production of antibodies by the B cells is also activated following the recognition of specific receptors on the MHC-antigen complex formed.

Acquired immunity is slow in response at first or initial exposure to foreign substance and invading microbes but very rapid in the second exposure or attack. The former is the primary response while the latter is secondary response. In primary immune response, an antigen or immunogen interacts specifically with components of the immune system such as antigen-specific B and T cells the first time. The innate immune response and other components of the immune system such as the cytokines and antibodies can also join forces with the primary response to initiate response to eliminate the invading microbe and/or foreign bodies that penetrated the body.

Secondary immune response is usually initiated when the same pathogen or a closely related immunogen that initially attacked the body resurfaced and is encountered by the host’s immune system the second time. In secondary response, the feedback of the immune attack against the invading microbe is usually rapid than the first (i.e. in the primary response) because the host had developed a memory of the immunogen, thus immunizing the individual to present a robust and rapid attack when its immune system encounters a closely related pathogen again.

It is noteworthy that an initial infection of a host with a particular pathogen initiates a state of memory or immunity which protects the individual against a possible second infection by the same or closely related microbe. This is the basis for the secondary immune response, which is a fundamental component of the adaptive (acquired) immune response and immunization. The four (4) qualities of the adaptive (acquired) immunity and which clearly differentiates them from the innate immunity shall be highlighted in this section.

  • Antigenic specificity: Adaptive immunity can clearly differentiate the differences that exist among immunogens i.e. it can tell apart between self molecules and non-self molecules.
  • Antigenic diversity: Adaptive immunity can recognize the numerous epitopes (antigenic determinants) that are found on the surfaces of immunogens and pathogens.
  • Immunological memory: Adaptive immunity can store the information about the invading microbe as memory in the first attack to show an enhanced response to a subsequent or second challenge by the same or a closely related microbe. Immunological memory allows adaptive immunity to confer a life-long protection or resistance to the host body against a wide variety of microbes after the first attack. This is the cornerstone for vaccination/immunization – a very important process in the medical sciences, and which has saved and is still saving untold number of people across the world from infectious diseases.
  • Immunological tolerance: Adaptive immunity possesses the ability to avoid making an adaptive immune response towards host molecules known as self. This implies that the adaptive immune response is capable of recognizing and distinguishing between self and non-self molecules; thus tolerating self molecules as much as possible to avoid the development of a disease or an abnormality in the immune system of the host.

There are basically four types of adaptive (acquired) immunity and these shall be highlighted in this section.

  1. Naturally acquired active immunity: Naturally acquired active immunity is the immunity acquired by an individual following prior exposure to an antigen or pathogenic microorganisms. This type of immunity is acquired by natural infections caused by pathogens inclusive of bacterial and viral agents. After the invasion of the host body by pathogens, the individual’s immune system is stimulated to produce antibodies, effector B or T lymphocytes and other immune response against the invading antigen; and naturally acquired active immunity can either be short-lived or it can last for a long period of time. Immunological memory stimulated by the initial entry of the antigen is also experienced in the affected host; and a rapid response is mounted against similar pathogen in future because of the memory B or T cells in the blood circulation. Humans who suffer from measles infection during childhood can no longer suffer from the disease or acquire its causative agent again in the future because their immune system have developed immunity against the disease during the first exposure of the individual to the pathogen; and this is a typical example of a naturally acquired active immunity. Newborns are given immunization against measles infection and a range of other viral and bacterial related diseases during the first few months of birth and booster doses are also given to ensure a longer lasting protection against pathogenic microorganisms.
  2. Naturally acquired passive immunity: Naturally acquired passive immunity is the immunity acquired when antibodies are transferred from one animal host to another especially from mother to child transplacentally. It is the type of immunity a child or newborn acquires from the mother through the placenta or breast milk. The transfer of immunoglobulins (particularly IgG) from mother to child or the foetus in utero confers a naturally acquired passive type of immunity on the neonate; and breast fed infants also acquire this type of immunity through breast milk (especially from the first part of the milk produced after delivery i.e. the colostrum). Colostrum is the secretion that accumulates in the mammary gland or breast of an expectant mother during the last weeks of her pregnancy; and this first breast milk released from the mammary gland after birth is rich in antibodies particularly secretory immunoglobulin A (sIgA) which confers local immunity against infection in the infant’s GIT. This type of immunity (i.e. naturally acquired passive immunity) is passive in nature because it does not last for a long period of time unlike the naturally acquired active immunity which last for a longer period of time. Naturally acquired passive immunity only last for few weeks or months (e.g. about six months in the neonate); and it protects the infant during these periods until the child can develop his own immunity against microbial infections and this usually occurs following exposure to pathogenic microorganisms or antigens.
  1. Artificially acquired active immunity: Artificially acquired active immunity is the immunity acquired by an individual after immunization or vaccination. In this type of immunity, the individual is given antigen preparations through injections; and this antigenic preparation (which is generally known as vaccines) is expected to spark the generation of numerous amounts of antibodies as well as effector lymphocytes that will protect the recipient host against any future exposure to pathogenic microorganisms. Vaccines are live or attenuated preparations of microorganisms which are generally used as antigens to confer immunity in living organisms (particularly humans and animals). Immunization is the administration of antigenic preparations of live or weakened microorganisms (i.e. vaccines or toxoids) especially through parenteral means to an individual in order to confer immunity against particular pathogenic microorganisms. And immunization/vaccination plays tremendous biological/immunological roles in the body especially in the activation and stimulation of the effector cells inclusive of memory B and T cells that mount rapid immunological response against invading antigens. Toxoids are inactivated nontoxic bacterial toxins which are used as antigens to spark immunity in the recipient animal host. Toxoids induce the production of antitoxins (i.e. antibodies that destroy or inactivate microbial toxins) in animal host when administered. Typical example of a toxoid vaccine is the one used for immunizing people against diphtheria and tetanus infection. Infants after birth are immunized against some pathogenic microorganisms including those that cause measles; tuberculosis and diphtheria amongst others up till the age of 15 years; and booster doses are given after this age limit in order to achieve a long lasting type of immunity in the individual. Bacilli-Calmette-Guerin (BCG) is a live attenuated vaccine used to vaccinate or immunize people against tuberculosis infection (caused by Mycobacterium tuberculosis).
  2. Artificially acquired passive immunity: Artificially acquired passive immunity is the immunity acquired when an individual is administered with specific antibodies or serum preparations containing specific antibodies or lymphocytes generated in another animal (e.g. horse). This type of immunity may spark allergic reactions in the recipient host since the antibodies are being generated from animal origin. Antiserum against tetanus and diphtheria as well as snake venom is mostly generated in horse or from equine origin. In artificially acquired passive immunity, specific antibodies against particular infections (e.g. tetanus) are generated in animals (e.g. horse) by first inoculating the animal with the antigen or pathogen; and the purified serum or immunoglobulin so generated is used as a medicinal or prophylactic measure to treat or prevent infection caused by that particular pathogenic microorganism in the recipient human host. People with wound infections and who have not been previously immunized against tetanus infections are given tetanus antiserum or horse antitoxin generated from horse as a therapeutic measure against the invading pathogen (i.e. Clostridium tetani). Artificially acquired passive immunity is not long-lived and thus it only last for few weeks or months.

HUMORAL/ANTIBODY-MEDIATED IMMUNITY (AMI)

Antibody-mediated immunity (AMI) also known as humoral immune response is an acquired or adaptive immunity that is generally mediated by immunoglobulins or antibodies and B lymphocytes. Humoral immune response mainly protects against extracellular bacteria, toxins and other extracellular foreign molecules. The B cells are mainly responsible for the production of antibody-secreting plasma cells and memory B cells; and both the B lymphocytes and the immunoglobulins are the main components of the AMI. After their stimulation, the B cells proliferate and differentiate into antibody-producing plasma cells which produce immunoglobulins that specifically binds antigens; and the memory B cells produced during this process ensure that antibody production occurs at a much faster rate in future i.e. if the host animal is attacked the second time by a similar antigen or antigenic molecule.

The interaction of B lymphocytes with pathogens or antigens is the main prerequisite for the proliferation and differentiation of B cells into plasma cells and memory B cells with a long lifespan than naïve B cells. The plasma cells secrete numerous antibodies (the main effectors of humoral immunity) during the activation of B cells; and the immunoglobulins secreted are generally the main effector molecules of AMI. Immunoglobulins continuously police the blood circulation in the body and they mark out or identify and neutralize trapped antigens in the process.

The coating of an antigen with antibody in vivo makes the foreign body to be easily attacked by other components of the immune system such as the engulfing of bacteria by phagocytes. The process of viral neutralization, opsonization, complement activation and phagocytosis are facilitated during immunological response when antigens are complexed with immunoglobulin molecules. The presence of immunoglobulins on mucosal surfaces including the GIT or intestinal tract, respiratory tract and nasal tract to mention but a few provides immunity to many infectious agents inclusive of bacteria and viruses; and deficiencies in the humoral immune response of a host may result in several microbial infections such as pyogenic infections that are bacterial-mediated i.e. caused by pathogenic bacteria.

IgM is the first antibody to be produced during primary immune response which is mainly characterized by the production of Ig-secreting plasma cells and memory B cells from the first contact of the host with exogenous antigens (Figure 16). In secondary immune response numerous amount of IgG is produced (Figure 17); and humoral immunity is generally responsible for providing defense against bacterial pathogens.

Figure 16: Schematic illustration of primary immune response. In primary immune response, immunoglobulin production is slow. Naïve or incompetent B cells undergo clonal selection after interacting with antigens to form antibody-secreting plasma cells and memory B cells. This phase of immune response (i.e. the primary immune response) is generally characterized by the initial production of numerous IgM which is later followed by the production of IgG. Primary immune response is a slower type of immune response which can last for a short time depending on the duration the invading antigen last in the host.

Figure 17: Schematic illustration of secondary immune response. Secondary immune response occurs weeks, months or years later following the exposure of the individual to the same antigen that invaded the body previously. Antibody production at this stage is mainly mediated by the memory B cells formed during the primary immune response; and the memory B cells undergo a rapid proliferation and differentiation into immunoglobulin-secreting plasma cells that ensures that enormous amount of antibodies (particularly IgG) is produced against the invading pathogen or antigen. The amount of IgM produced in secondary immune response is usually lower when compared to the amount of IgM produced in primary immune response; and the high amount of IgG produced in secondary immune response ensures that immunological response occurs at a much faster rate than the slower reaction obtainable in the primary immune response.

ANTIGEN PROCESSING AND ANTIGEN PRESENTING CELLS

Antigen processing is the immunological process involved in the digestion or breakdown of antigenic molecule to release peptide molecules that are associated with MHC molecules and presented on the surfaces of the infected host cells for presentation to effector lymphocytes particularly the T cells. It is the series of immunological events that is mainly involved in the formation of antigenic peptide molecules complexed with MHC molecules (Class I and Class II MHC molecules inclusive) for rapid immunological response. The ability of the immune system to process an antigen and make it available for further specific immunological response is vital to the elimination of the foreign body; and antigen processing is important for the elicitation of an appropriate immune response. Depending on their portal of entry into the body, the antigen processing arm of the immune system ensures that immunogens or antigens are properly presented to the effector molecules of the immune system for an enhanced immune response against them. Antigen processing help to unfold intracellularly hidden parasites on the surfaces of infected host cells.

Macrophages which also act as antigen presenting cells are typical examples of antigen processing cells of the immune system that parades the blood circulation or body looking for antigens which they ingest and digest to produce antigenic peptide molecules that are recognized by effector T lymphocytes. Generally, antigen processing is required to produce antigenic peptide molecules that interact with specific MHC molecules. While antigen processing is mainly involved in the formation of antigenic peptide molecules complexed with MHC molecules, antigen presentation is the sequence of immunological reaction involved in the display of antigenic peptide molecules in complexed with MHC molecules on the surfaces of antigen processing cells (APCs).

After their entry into the body, antigens or pathogens are ingested and degraded by antigen processing cells of the immune system including the macrophages; and peptide fragments from the degraded antigen is displayed on the cell surface of the macrophages in association with MHC molecules either Class I MHC molecules or Class II MHC molecules. It is noteworthy that the Class I MHC molecules present antigens to T-cytotoxic cells (CD8+) while Class II MHC molecules present antigens to T-helper cells (CD4+). The main biological function of the Class I and Class II MHC molecules is to bind antigenic peptide molecules or fragments from degraded antigens and bring them to surfaces where they can be recognized by the appropriate T lymphocytes especially the CD4+ and CD8+ cells.

APCs are specialized cells of the immune system whose main function is to ingest, process, and present processed antigens in association with any of the MHC molecules to specific arms of the immune system especially the T lymphocytes for further immunological response. Some pathogenic microorganisms including but not limited to viruses, protozoa and some obligate intracellular bacteria (e.g. Chlamydia) express their virulence or pathogenicity within their infected host cells where some components of the immune system (e.g. immunoglobulins) cannot reach for immunological response. For such intracellularly-placed pathogens to be attacked by the immune system, the MHC molecules which is normally located on the cell surfaces of normal host cells inclusive of infected host cells displays peptide fragments resulting from the degradation of the intracellular parasite; and such peptide-MHC complexes are the antigens to be recognized by effector molecules of the T lymphocytes especially the CD8+ cells whose main function is to kill viral infected cells and other intracellular parasites.

Such antigens are generally referred to endogenous antigens because they resulted from cells infected by pathogens that live inside host cells (e.g. viruses and protozoa). Those antigens that do not invade host cells (e.g. pathogenic bacteria) but end up in the lymph nodes where they could easily be attacked or ingested by phagocytes and other components of the immune system such as antibodies are generally called exogenous antigens because they do not reside within host cells.

Unlike the B cells can recognize local antigens (i.e. pathogens that do not reside within the host cells) without the assistance of the MHC molecules; the T lymphocytes cannot recognize antigens on their own unless such antigens have been processed by APCs and presented in conjunction with any of the MHC molecules either Class I or Class II MHC molecules. Therefore, the processing and presentation of antigens by the APCs is critical for the recognition of the immunogen by the T cells. Macrophages, dendritic cells and B cells are typical examples of antigen presenting cells (APCs); and these APCs recognize, bind or capture and display processed antigenic molecules on their cell surfaces where other specific components of the immune system such as the T lymphocytes can effectively neutralize them.

They are generally known as professional APCs because the display peptide fragments (from antigens) complexed with Class II MHC molecules to T helper (TH) cells. This is important in immune response to a particular antigen because TH cells help B cells to proliferate and secrete numerous antibodies; and this further elaborate the immunological synergy that exist between the humoral or antibody mediated immunity (AMI) whose main effector molecule is the B cell and the cell-mediated immunity (CMI) whose main effector is the T lymphocytes. Non-professional APCs which include thymic or thyroid epithelial cells, skin fibroblasts and brain grail cells amongst others are APCs that are usually stimulated to express Class II MHC molecules on their cell surface membranes; and non-professional APCs only last for a short period of time in an immune response especially in inflammatory reactions in the body.

PHAGOCYTOSIS

Phagocytosis is simply the immunological process mediated by some immune system cells such as macrophages and neutrophils engulf or ingest bacteria and other particulate materials. These cells of the immune system whose main function is to eat or engulf bacteria are generally known as phagocytes. Phagocytes attack and engulf exogenous bacteria and other extracellular particles; and they also ingest and digest other insoluble particles and endogenous materials. The biological significance of phagocytosis in immune response was first explained by Elie Metchnikoff (1845-1916), a Russian scientist who coined the phrase “phagocytes” to describe some type or group of white blood cells (WBCs) that engulf or eat pathogenic bacteria and other large complex molecules during phagocytosis.

Phagocytosis is a significant biological phenomenon during immune response; and it helps to localize and restrain pathogenic bacteria and other foreign bodies that invaded the host from further expression of their pathogenic activity in vivo. Though a well established innate immune response, phagocytosis also facilitates the cell-mediated immunity since some phagocytic cells (e.g. dendritic cells) helps in antigen presentation to the MHC complex molecules i.e. Class I MHC and Class II MHC molecules depending on the type of antigenic molecule being processed.

The process of phagocytosis ensures that ingested bacteria and other particulate materials are properly digested by specialized enzymes (e.g. lysozymes) and then released or eliminated from the body. Phagocytosis is generally an innate immune response that ingests and degrades bacteria and other extracellular materials. During phagocytosis, the plasma membrane of the phagocytic cells (e.g. macrophages) expands and engulfs the microorganism to form a large vesicle known as the phagosome; and degradative enzymes (e.g. lysozymes) present in the phagosome ensures that the ingested bacteria is efficiently degraded or digested before being eliminated from the host’s body. Phagocytosis is a multifaceted immunological process that involves several processes and these stages of phagocytic action during immune response shall be elucidated in this section.

HYPERSENSITIVITY REACTION (ALLERGY)

Hypersensitivity is a condition that causes the body to respond very strongly especially in an undesirable manner to allergic substances or allergens. It can also be referred to as allergy or allergic reaction. Though they can be generally classified as exaggerated in vivo reactions mediated by the immune system to foreign bodies or antigens that entered the body and, immunological responses that are not normally harmful to the host; hypersensitivity reactions may destroy the host cell in the process of destroying the pathogen or allergen. Allergens are substances that provoke hypersensitivity reactions or allergy in an individual. Allergic substances include dust, pathogens, foods (e.g. nuts, egg and milk, sea food and some beans), venoms of some insects such as bees and wasps, drugs (e.g. penicillin and sulphonamides), animal hairs and even pollen grains from some flowers (e.g. poison oak plant).

These allergic substances enter the body via several routes especially through the mouth and nose or nares that are known to be rich in mucous substances. Immunoglobulin E (IgE) is amongst the antibodies that colonize the mucous membranes of these surfaces (aside secretory IgA); and the level of IgE in the serum of people exposed to allergens (e.g. parasites) is usually on the increase compared to normal individuals without exposure to allergic substances. This is because IgE is the primary antibody produced by the immune system against allergens; and IgE is generally known as reagenic antibody because it is the major immunoglobulin involved in anaphylactic reactions – in which the immunoglobulin binds to and brings about the degranulation of mast cells or basophils to produce pharmacologically active substances (e.g. histamine) that cause hypersensitivity reactions in the host.

Individuals react differently to different allergic substances; and allergens generally stimulate adverse immunological response in individuals who come in contact with them. In hypersensitivity reactions, the host’s immune system is provoked (especially by allergens) to act in an exaggerated fashion which is harmful to the body. Depending on the type of invading allergen, some allergic or hypersensitivity reactions could be immediate-type hypersensitivity or delayed-type hypersensitivity. While delayed-type hypersensitivity reactions occur slowly following the introduction of allergens into the body, the immediate-type hypersensitivity occur in a much faster or rapid manner after the exposure of the sensitized individual to a further dose of the allergen. An individual becomes immunologically sensitized after its first contact with an antigen (in this case an allergen); and this illustrates the primary response of the host’s immune system to the antigen – in which antibodies are produced against the invading antigen.

However, when the same host or individual comes in contact with the same allergen or antigen the second time, there is a heightened immunological response known as a secondary response – in which the allergic response or reaction actually occurs. In some cases, the secondary immunological response to the invading allergen or antigen may be excessive; and this phenomenon leads to the production of effector cells of the immune system that stimulate mild subclinical and localized inflammatory reaction in the body. Though such localized inflammatory response may be beneficial in protecting the host organism from the adverse effect of the invading pathogen; the response may become severe and out of control in such a way that it affect the body adversely, leading to the damage of the host’s tissues and even death in some cases. The biological processes leading to the damage of the host’s tissues when immunological responses (inclusive of the B and T cell response) to recognized foreign bodies or antigens that invaded the body generally results to an immunological response known as hypersensitivity reaction or allergy. There are mainly four types of hypersensitivity reactions, and these shall be highlighted in this section.

  • Type I hypersensitivity
  • Type II hypersensitivity
  • Type III hypersensitivity
  • Type IV hypersensitivity

Type IV hypersensitivity is generally referred to as delayed-type hypersensitivity because this type of hypersensitivity reaction involves mainly the cell-mediated immunity which may usually take a longer period of time to become apparent. Delayed-type hypersensitivity (DTH) reactions usually occur days or weeks after the body becomes sensitized following the introduction of antigens into the host. Immediate-type hypersensitivity includes Type I, Type II and Type III hypersensitivity because these types of hypersensitivity reactions mainly involves the humoral or antibody-mediated immunity in which immunoglobulins are produced instantaneously upon the body’s encounter with antigens. Unlike the DTH reaction whose symptoms normally take days to occur, the symptoms of the immediate-type hypersensitivity reaction occurs minutes to hours after the exposure of the host to the allergens or antigens. Hypersensitivity reaction is generally a heightened condition of immune responsiveness that causes damage to the host. 

TYPE I HYPERSENSITIVITY

Type I hypersensitivity which can also be called anaphylactic or atopic hypersensitivity reaction is an IgE-mediated type of allergy that occurs immediately following the exposure or prior sensitization of the host’s body by the invasion of an allergen. It generally occurs when an allergen binds to the surface of an IgE molecule whose FC region or receptor is attached to a mast cell or basophil; and this binding leads to the degranulation of the mast cells to release pharmacologically active substances (e.g. histamine and heparin) that causes adverse inflammatory reactions in the affected individuals. There is usually a short time observed between the exposure of a sensitized human host to an allergen and the appearance of clinical symptoms associated with anaphylactic reaction; and this is why Type I hypersensitivity is generally regarded as an immediate-type hypersensitivity reaction.

Mast cells are immune system cells that are derived from the bone marrow cells during haematopoiesis; and they are present in several tissues of the body (especially in connective tissues) where they bind to the FC region of IgE molecules to mediate inflammatory response as is obtainable in hypersensitivity reactions. They are cells that bind IgE; and the effector molecule of Type I hypersensitivity is IgE. Mast cells contain numerous cytoplasmic granules that are shed following the cross-linking of an IgE molecule with an allergen in a human or animal host; and they have receptors for the FC region of immunoglobulin E on their cells surfaces. Type I hypersensitivity is an immediate-type allergic reaction; and this type of hypersensitivity reaction can be localized or systemic/generalized inflammatory response depending on the antigenicity of the invading allergen(s) amongst other factors.

In Type I hypersensitivity reaction, the host becomes sensitized to a given allergen following an initial introduction of the antigen into the host’s body. The host’s immune system produces numerous IgE against the invading allergen at the first exposure to the antigen; and the IgE molecules produced becomes attached via their FC region to receptors on the mast cells or basophils. The IgE so produced parades the general circulation in search of allergens to bind to. After some days or months past the first exposure and the host becomes exposed to the same allergen the second time, the allergen cross-links the IgE molecules on the mast cells or basophils (i.e. the IgE attaches the allergen to the mast cells), and this result in the production of chemical or physiological or vasoactive mediators such as histamine and prostaglandins amongst others which causes an immediate anaphylactic reaction in the affected animal host (Figure 18). The degranulation of the mast cells or basophils during anaphylactic reaction is mainly mediated via the cross-linking of the IgE molecule(s).

Figure 18: Illustration of Type I hypersensitivity reaction. IgE is the main mediator of Type I hypersensitivity or anaphylactic reaction; and the main biological function of this immunoglobulin molecule in anaphylactic reaction is to cross-link (i.e. act as a bridge) between the allergen that invaded the body and the mast cells or basophils which are known to contain cytoplasmic granules (e.g. histamine). The FC region of the IgE molecules binds specifically to the receptors on the mast cells or basophils while its Fab region or fragment binds to the allergen; and this binding leads to the degranulation of the mast cells to release vasoactive substances such as histamine, heparin and prostaglandins amongst others which ultimately provoke the clinical or subclinical conditions associated with the Type I hypersensitivity reaction.

Diarrhea, urticaria, vomiting, eczema, body itching, asthma, hay fever, and food allergies are some examples of Type I hypersensitivity reaction. However, in severe or life-threatening anaphylactic reactions (e.g. disintegration of the vascular system) death can occur unless appropriate medical help is rendered to the affected human host. Type I hypersensitivity reaction is the commonest type of allergic reactions in humans. It is worthy of note that the physiological mediators produced in anaphylactic reactions also mediate a protective inflammatory reaction in the host, but in severe responses these mediators could cause harsh medical condition (i.e. anaphylactic reactions) in the host. The pharmacologically active substances released in Type I hypersensitivity reaction can also cause the vasodilation or constriction of smooth muscles; and this may affect the normal physiological functions of some vital muscular activities in the affected hosts especially in respiratory activities such as in asthma sufferers whose allergen entered the body via the respiratory tract by inhalation.

Allergens that provoke anaphylactic reaction can also enter the body via foods (i.e. by ingestion) and through direct inoculation or drug administration especially parenterally. Microbial spores, some foods especially nuts and proteinous foods, insect venoms, drugs (e.g. penicillin and sulphonamides), pollen grains from plants or flowers and dust particles are some examples of allergens that could provoke anaphylactic reactions in human or animal hosts. The clinical outcome or symptoms of Type I hypersensitivity reaction is usually affected by some host factors and other environmental conditions including but not limited to the amount of the administered allergen, route of administration and the number of times the host is exposed to the antigen or allergen. Type I hypersensitivity reaction could be prevented especially by avoiding contact with the allergen(s) responsible for the anaphylactic reaction. Anti-histamine drugs which block the release of histamine; and other clinical remedy could also be applied in containing the adverse effects of anaphylactic reactions in affected human hosts. In life-threatening cases, it is also critical to ensure appropriate airflow in the affected individual in order to maintain proper respiratory function of the affected human host.

TYPE II HYPERSENSITIVITY

Type II hypersensitivity (which can also be referred to as a cytotoxic allergic reaction) is an immediate-type hypersensitivity that is mainly observed in blood transfusion reactions and haemolytic disease of the newborn (HDN). The effector molecules of Type II hypersensitivity reaction are IgG and IgM antibodies. Type II hypersensitivity reaction can also be called a cytolytic allergic reaction because it often result in the destruction or killing of host cells. In Type II hypersensitivity, the antibody-mediated arm of the immune system is stimulated or provoked to produce antibodies against host self-molecules or receptors found on the membranes of host cells. It is an antibody-mediated destruction of host cells in which immunoglobulins directed against antigens or allergens activate the complement system to cause complement-mediated lysis of host cells.

Activation of complements can mediate ADCC or it can create holes on the cell membrane of the invading antigen; and cytotoxic cells with receptors for FC region of antibodies binds to the FC region of the produced immunoglobulins (mainly IgM and IgG) already attached to target cells and this promotes the killing of host cells. Type II hypersensitivity reaction can also be observed in some autoimmune diseases (e.g. Graves disease) and in tissue or organ transplants in which the grafted organ is rejected because the recipient host have preformed immunoglobulins against the grafted tissue or organ; and this ultimately result in the rejection of the transplanted organ few hours or days after transplantation.

The cell surface of red blood cells (RBCs) or erythrocytes of humans is normally lined with specialized antigens known as the ABO blood groups antigens which are generally known as glycoproteins found on the cell membrane of erythrocytes. It is the ABO blood groups antigens that is used to elucidate the particular blood group of an individual in blood group typing in the laboratory; and every individual possess naturally occurring antibodies known as isohaemagglutinins – which are produced against foreign blood group antigens not found on the cell surface of the host’s RBCs. The ABO blood groups antigens of humans are normally encoded by different genes that belong to multiple allelic groups; and since an individual with an allelic form of blood group antigen can also recognize other allelic forms of blood groups on transfused blood, the recipient host will immediately mount an immunological response against the foreign blood group antigen.

This phenomenon is always taken into consideration in clinical medicine whenever blood transfusion between two different donors and recipients is contemplated, and in order to prevent transfusion reaction, it is critical to know the compatibility of the recipient and donor blood groups via ABO blood group typing or determination. An individual with group A blood has anti-B antibodies (i.e. isohaemagglutinins) and the reverse is also the case for persons with group B blood who has anti-A antibodies. Persons with the AB blood group have neither anti-A antibodies nor anti-B antibodies while people with blood group O possess both anti-A antibodies and anti-B antibodies. People with the AB blood group are known as universal recipients because they lack the anti-A and anti-B antibodies and thus can receive blood from other blood groups while those with the O blood group are known as universal donors because they can donate blood to other blood groups since they possess the anti-A and anti-B antibodies.

Transfusion reaction, a Type II hypersensitivity reaction mainly occur when mismatched or incompatible blood is transfused; and this results to a clinical condition in which the donors RBCs becomes rapidly coated with the recipients isohaemagglutinins thus activating the complement system of the host or recipient in an adverse manner. The ABO blood groups antigens of humans and their respective isohaemagglutinins is shown in Table 1. Typical example of a transfusion reaction occurs when an individual with blood group B (the recipient) receives blood from a person with blood group A (the donor). In such scenario, the anti-A isohaemagglutinins of the recipient host rapidly coats the group A blood cells of the donor, and this mediate a cytolytic reaction in which the donors blood cells are destroyed or haemolyzed via complement activation (i.e. complement-mediated lysis) and the action of antibodies. Haemoglobinuria (i.e. the presence of haemoglobin or blood in urine), fever, clotting of blood in blood vessels and kidney problems are some clinical manifestations of transfusion reaction in man.

Haemolytic disease of the newborn (HDN) also known as erythroblastosis fetalis is a Type II hypersensitivity reaction in which the maternal antibodies (specifically IgG) produced against fetal Rhesus (Rh) antigens or RBCs crosses the placenta to cause haemolysis in the neonate (i.e. the destruction of fetal RBCs). Erythroblastosis fetalis shows the Rh factor incompatibility that exists between the mother and her foetus or unborn child; and this clinical and/or immunological reaction is mainly experienced in unborn foetus or neonates whose mothers have been previously exposed to blood group antigens on the RBCs of the foetus especially in their first pregnancy. Typical example is when the mother is Rhesus factor negative (Rh) and the foetus is Rhesus factor positive (Rh+). The mother produces anti-Rh antibodies against the fetal blood, and the antibody crosses the placenta to cause haemolysis of fetal RBCs thus causing severe or mild anaemia depending on the severity of the disease.

Table 1: ABO blood groups antigens of humans

Blood groups

(Phenotypes)

Genotypes *Isohaemagglutinins Agglutinins (antigens on RBCs)
A AA or AO Anti-B A
B BB or BO Anti-A B
AB AB None A and B
O OO Anti-A and Anti-B None
*Isohaemagglutinins comprises mainly of the immunoglobulin M (IgM) class.

Most people are Rh+. However, during delivery (especially in a first pregnancy) there is a mixture of fetal red blood cells (i.e. a Rh+ fetal blood) with that of the mother (a Rhmother) during delivery and this provoke the mothers immune system to produce antibodies (inclusive of IgM and IgG) whose function is to clear the maternal blood circulation from every trace of fetal blood. At this stage the mother’s immune system is sensitized and memory B cells will be produced to counter future Rh+ fetal RBCs. In the first pregnancy, the Rh mother is not exposed to enough fetal blood (usually Rh+ blood cells) in order to activate her Rh specific B cells against possible futuristic Rh+ RBC from neonates in subsequent pregnancy. The first child is usually not affected when the blood of the foetus leak into the blood circulation of the mother.

HDN is mainly experienced in subsequent pregnancies especially when the mother is not clinically treated against the disease condition. The memory B cells produced in the mother remain in the maternal circulation to produce antibodies against the fetal Rh+ blood in subsequent pregnancy. In subsequent pregnancy, the memory B cells become activated in a rapid fashion to produce anti-Rh antibodies (mainly of the IgG class) that cross the placenta to haemolyze or lyse the fetal red blood cells i.e. if the ensuing foetus is Rh+. Activation of the complement system mediated by the memory B cells in the mother causes erythroblastosis fetalis in the neonate; and this is usually observed clinically as mild or life-threatening anaemia in utero.

The Rhesus status of the newborn and that of the parents are illustrated in Table 2. Erythroblastosis fetalis occurs in cases of Rhesus incompatibility i.e. in clinical conditions in which the mother is Rh and the foetus is Rh+. However, passive immunization of the mother via parenteral administration of antibodies (e.g. RhoGAM) against the Rhesus antigens to the mother few hours after her first delivery (e.g. 24-72 hours) prevents Rhesus incompatibility and thus erythroblastosis fetalis in subsequent pregnancies. Neonates in subsequent pregnancies will be prevented from possible HDN because the RhoGAM (anti-Rh antibodies) binds any fetal blood that entered the mother’s blood circulation during delivery in order to clear the maternal blood from fetal Rh+ blood prior to B cell activation and memory B cell production. Immunized mothers are not likely to produce IgG anti-Rh antibodies that will cross the placenta to cause the haemolysis of fetal blood; and it is advisable that Rh mothers be properly immunized after delivery so that they will not generate Rh specific memory B cells in the mother.

Table 2: Haemolytic disease of the newborn (HDN) and Rhesus status

Rhesus type Haemolysis
Father Mother Child
+ + + or — No haemolysis
 

+

 

+ No haemolysis in first child. But haemolysis occurs in second child and subsequent deliveries. Parenteral administration of anti-Rh antibodies to an Rh(D) mother after the delivery of a Rh(D)+ child helps to prevent possible development of HDN.
+ No haemolysis
+ + or — No haemolysis
No haemolysis
Aside the agglutinogens (i.e. the ABO blood group factors) in which human blood is mainly classified, the Rhesus antigen or factor usually expressed as “Rh(D) antigen” is another basis on which human blood can be classified. Rhesus factor is an antigen found in red blood cells (RBCs), and it is a crucial factor considered in human blood grouping techniques. Each of the human blood groups including blood groups A, AB, B and O can either be Rhesus negative Rh(D) or Rhesus positive Rh(D)+; and a detrimental antigen-antibody reaction is bound to occur when a Rh(D) individual is transfused with a Rh(D)+ blood. Similar adverse antigen-antibody reaction as aforementioned also occur when a mother that is Rh(D)has a foetus or neonate whose blood group is bearing the Rh(D)+ antigen probably inherited from the father (that is Rh(D)+). The Rh(D)+ blood of the foetus crosses the placenta of the mother during the delivery of the first child (harbouring Rh(D)+ blood) and enters her blood circulation where it stimulate the production of antibodies (specifically anti- Rh(D)+ antibodies).

In subsequent deliveries (especially in Rh(D)+pregnancies), the anti- Rh(D)+ antibodies of the mother produced in the first delivery crosses the placenta and enters the blood circulation of the neonate or foetus, and this results in erythroblastosis fetalis otherwise known as haemolytic disease of the newborn (HDN). In HDN, there is a rapid lysis or destruction of the neonate’s RBCs. In summary, HDN is a blood disease that affects neonates, and it is mainly caused by an immunological reaction that occurs between the Rhesus factor of the mother and that of the neonate (i.e. if the foetus is Rh(D)+. Most individuals are Rh(D)+. And a Rh(D)person should not be transfused with a Rh(D)+  blood to avoid the development of antibodies against the blood in the individual. Rhesus factor compatibility is a key factor taken into consideration during blood transfusion and even in pregnant or expectant mothers especially those that are Rh(D).

TYPE III HYPERSENSITIVITY

Type III hypersensitivity also known as immune-complex allergic reaction is mainly characterized by an adverse inflammatory response mediated by antigen-antibody complexes. Antibodies react specifically with antigens to form small soluble immune-complexes which are finally cleared from the system by phagocytic cells, macrophages and the reticuloendothelial system (RES). The formation of immune-complexes in normal antigen-antibody reaction makes the invading pathogen easily opsonized and phagocytosed by phagocytes. However, in some antigen-antibody reaction the immune-complexes formed cannot be eliminated from the host’s body (due to the large size of immune-complexes formed); and these large insoluble immune-complexes (that usually comprises of IgM and IgG) find their way into various tissues of the body where they cause several tissue damages via the release of several proteolytic enzymes and other tissue-damaging substances.

IgM and IgG are the main effectors of Type III hypersensitivity reaction. The skin tissues, glomeruli (singular: glomerulus) of the kidney and the host’s blood vessels are some examples of the tissues that can be affected by the deposition of immune-complexes. Deposition of antigen-antibody complexes in the body’s tissues activates several components of the immune system especially the complement system; and the attempt by the polymorphonuclear (PMN) cells (e.g. neutrophils) to eliminate these immune-complexes result in inflammatory responses and tissue damage. Type III hypersensitivity reaction is mostly common amongst antigens or pathogens (e.g. viruses, bacteria and parasites or protozoa) whose pathogenicity in vivo is usually associated with some level of damage to their host tissues or cells. Immune-complex mediated allergic reactions i.e. Type III hypersensitivity could cause a localized reaction or systemic reaction depending on the route of entry of the causative antigen or allergen; and they generally occur when the immune system of the animal host becomes inundated by large insoluble immune-complexes.

Arthus reaction, a localized Type III hypersensitivity and serum sickness, a generalized or systemic Type III hypersensitivity are typical examples of immune-complex allergic reactions. Serum sickness is characterized by fever, arthritis, itching, oedema and rashes; and it occurs few days after large amounts of allergens are inoculated or injected into the bloodstream. In Arthus reaction, the allergen is injected intradermally or subcutaneously at high concentrations; and it is usually characterized by tissue necrosis and other inflammatory reactions at the site of administration. Type III hypersensitivity is also implicated in some clinical conditions including autoimmune diseases (e.g. rheumatoid arthritis), some microbial diseases (e.g. post-streptococcal glomerulonephritis), systemic lupus erythematosus and allergies to some drugs such as penicillin and sulphonamides.

TYPE IV HYPERSENSITIVITY

Type IV hypersensitivity which can also be called delayed-type hypersensitivity (DTH) reaction is a cell-mediated allergic reaction that induces a localized inflammatory reaction via the release of cytokines. In delayed-type hypersensitivity, sensitized T lymphocytes mediate the release of cytokines (e.g. interleukins and interferons) that recruit macrophages to the site of infection or allergen administration. The recruited and activated macrophages release lytic enzymes that cause localized tissue damage at the affected body site as well as contain the activities of the invading allergen. Sensitized T lymphocytes (particularly TDTH cells) are the main effector molecules of the Type IV hypersensitivity reaction and macrophages also act as effector cells of DTH response.

Immunoglobulins and complements play no roles in delayed-type hypersensitivity reaction as is obtainable in Type I, II and III hypersensitivity reactions. Unlike other forms of hypersensitivity reactions (inclusive of Type I, II and III reactions as aforementioned) that are immediate-type allergic reactions and occur soon after the injection of the allergen into the host; the Type IV hypersensitivity reaction occurs hours or days after the allergen or antigen have invaded the animal host and thus the name delayed-type hypersensitivity.

There are plethora of pathogenic microorganisms that induce a delayed-type hypersensitivity reaction and these include: intracellular bacteria (e.g. Mycobacterium tuberculosis, Brucella abortus, Listeria monocytogenes and M. leprae), intracellular viruses (e.g. measles virus, herpes simplex virus and small pox virus or variola), intracellular protozoa (e.g. Leishmania species), intracellular fungi (e.g. Candida albicans, Cryptococcus neoformans, and Histoplasma capsulatum), and other contact allergens such as poison ivy and poison oak that causes several skin reactions in sensitized humans. Delayed-type hypersensitivity (DTH) unlike other forms of hypersensitivity reactions has several protective roles in the animal or human host aside causing localized tissue damage.

For example, Type IV hypersensitivity defends the host against intracellular microorganisms and other contact allergens; and the recruitment and activation of macrophages in the Type IV hypersensitivity reaction to the site of infection is critical in defending the body against intracellular microorganisms which are known to live inside the host cell and away from possible attack by antibodies and other components of the host’s immune system. The intracellular pathogen is eliminated with little damage to the host cell but if the allergen or antigen is not easily removed, an extensive DTH reaction may result in adverse inflammatory reaction that is characteristic of Type IV hypersensitivity reaction.

Contact dermatitis caused by direct body contact with some plants or flowers (e.g. poison ivy and poison oak tree) is a typical example of delayed-type hypersensitivity reaction. In the hospital, skin testing (e.g. Mantoux test) based on DTH response is used to determine the sensitivity or sensitization of an organism’s immune system to the invasion of an allergen or antigen (in this case: exposure to the tubercle bacilli, Mycobacterium tuberculosis).

BIOLOGY OF TRANSPLANTATION

Transplantation is simply the medical procedure of replacing an abnormal or diseased/damaged cell, tissue or organ by a functional and normal one. Normal tissues or organs are usually obtained from donors (living or deceased) and transplanted to the recipient hosts; and this surgical and immunological procedure is life-saving in the sense that there is a restoration of the normal physiological function of a damaged organ or tissue in the recipient host(s). In transplantation immunology, the transferred organ or tissue (i.e. the one emanating from the donor host) can also be referred to as graft. Grafts and transplantation can sometimes be used synonymously too.

Cells or tissues can be grafted from one part of the body to another part and also between two different or related animal hosts. Blood cells, cornea, heart, bone marrow cells, kidney, and skin cells are some examples of the types of tissues, cells or organs that are often transplanted in humans. Despite their clinical significance especially in the restoration or re-establishment of the normal functioning of a given organ or tissue; transplantation is associated with some untoward effects which sometimes thwart its purpose in the recipient host. Typical amongst these setbacks of transplantation or graft is tissue or organ rejection.

The rejection of a grafted or transplanted tissue or organ by the recipient host is one of the major problems of organ transplants. As aforementioned, every individual (except in identical twins) have a unique human leukocyte antigen (HLA) or major histocompatibility complex (MHC) molecule that lines their cell membranes; and the uniqueness of the HLA/MHC molecules causes the rejection of grafted tissue because the body of the recipient host sees these grafted tissues or organs as foreign and thus mount an immunological response against them.

Cellular and antibody-mediated immune responses as facilitated by the T lymphocytes and antibodies respectively are immediately stimulated to mount an attack against the grafted tissue or organ; and this is mostly experienced in cases where tissues, cells or organs are grafted between unrelated hosts. Tissue or organ rejection in transplants is less-likely in individuals that are closely or genetically-related. Transplants between identical twins also have a lesser possibility of undergoing rejection.

When the HLA/MHC molecules of the donors and the recipients are not identical, the recipient’s cellular and humoral immune response sees the incoming tissue or organ as foreign and thus the recipient’s immune system mounts an immunological attack against the grafted tissue, thereby leading to its rejection. HLA/MHC typing prior to tissue/organ transplant in the laboratory is critical to identify the genetic-relatedness of the donor and recipient cells (i.e. the closest HLA/MHC match between the donor and recipient hosts), and this practice has helped to save a variety of transplantation procedures in humans and animals alike. Recipients and donor should have similar HLA/MHC molecules for transplantation to be successful.

To prevent rejection clinically, some medical procedures or techniques are also available to ensure tolerance of the grafted tissues/organs by the host’s body. Livelong immunosuppressive drugs for example have been used clinically to prevent the rejection of grafts in recipient hosts; and these measures help to ensure the survivability of the grafted tissues. These immunosuppressive drugs and/or anti-inflammatory drugs used during transplantation (e.g. cyclosporine, corticosteroids, antiserums and antimetabolites amongst others) help to induce the tolerance of the recipient’s body to the grafted tissue/organ. However, these immunosuppressive or anti-inflammatory drugs helps to calm the recipient’s immune system and thus make it to be more tolerant to the grafted tissues, cells or organs which it sees as foreign or non-self molecules.

In an immunological tolerant state especially when immunosuppressive drugs are used during transplantation as aforementioned to prevent possible rejection of the grafted tissues or organs, the immune system of the recipient organism is taught to become amenable and unresponsive to alloantigens and the grafted tissues thereby extending the viability of the graft. The rejection of a grafted tissue or organ generally leads to a clinical condition known as graft versus host disease (GVHD). Graft versus host disease (GVHD) or reaction is a medical condition that develops when cells from the grafted tissue or organs (i.e. from the donor host) react against or with the recipient’s own tissues leading to an immunological response that facilitates the rejection of the transplanted tissue/organ. GVHD (which usually occur in bone marrow and liver transplants) is mainly mediated by the cell-mediated immunity and the T cells play a critical role in this immunological reaction. The T cells from the graft or transplant recognizes the alloantigens of the host cells i.e. their MHC molecules as foreign and thus mounts an immunological response against it.

Alloantigens are unique antigenic molecules found on the surfaces of host cells and they vary among the individual members of a given species. They are antigens that are found in another member of the host’s species; and these antigens are capable of provoking an immunological response in the host. Alloantigens are important decisive-factors to be considered in transplantation and even in blood transfusions since they are unique self-molecules of the host and differ amongst members of the same species. GVHD is a Type IV hypersensitivity reaction; and it occurs mainly in immunocompromised individuals receiving immunocompetent cells. The tissues of the recipient host are attacked in a GVHD condition by the transfused cells of the donor host few days later or after the transplantation procedure. The mechanism of graft rejection in human or animal hosts is usually characterized by two stages viz: the sensitization stage and the effector stage; and both of these phases of tissue rejection are responsible for the rejection of grafted tissues or organs.

Sensitization stage is the first phase in the rejection of transplanted tissues, cells or organs; and in this stage of graft rejection the immune system of the recipient host becomes sensitized by the grafted tissues (in response to allergens in particular) and this stimulate an immunological response especially the T cells to proliferate into effector molecules which mounts a rejection reaction at a later stage. In the effector stage, a variety of immunological mechanisms including but are not limited to, ADCC, TDTH reactions and cytolytic activities mediated by T cytotoxic TC) cells. At this stage (i.e. the effector stage of tissue rejection), activated macrophages and T lymphocytes sensitized at the sensitization stage migrate in a rapid fashion to the site of graft and this mechanism facilitates the outright rejection of the grafted tissues, cells or organs. During the effector stage of graft rejection, chemokines and cytokines in particular including the interleukins, TNFs and interferons play a critical role in facilitating graft rejection and this is because these molecules help to stimulate the proliferation of cytotoxic T cells which eventually carryout targeted killing of cells associated with the grafted tissue or organ.

Graft rejection can be acute, chronic or hyperacute in nature depending on the level of immune response mounted on the grafted tissue or organ. In acute rejection, there is an infiltration of lymphocytes and monocytes to the graft and this rejection which occurs few days after transplantation is mainly mediated by T cells. Chronic rejection occurs months or years after grafting and it is mainly mediated by antibodies, cytokines and complements. In hyperacute rejection, grafted tissues or organs are rejected immediately (i.e. few minutes or hours) after transplantation; and this type of rejection is mainly mediated by preformed antibodies in the recipient host which recognizes alloantigens or antigens of the donor host. There are four major types of graft or tissue/organ transplantation that can occur in humans or animals and they include autograft, isograft, allograft and xenograft (Table 3). The level of immunological response to a graft is usually dependent on the type of transplant or graft; and the classification of graft into these four categories is also based on the genetic relatedness or relationship that exist between the donor host and recipient host.                    

Table 3: Types of grafts

Type of graft Clinical connotation Level of rejection
Autograft This is a self-tissue graft in which tissues obtained from a given individual or donor is transplanted back into the same donor. Autograft is usually used in cases where tissues are grafted from one location of the body to another location on the same host. Typical example is the grafting of skin tissues from the thigh to restore the normal functioning of another part of the skin (e.g. face or arms) that is severely damaged due to burns. This type of grafting can also be called autologous grafting or transplant because graft was taken from one location in the donor and returned to the same donor at a different body site. Autograft is often employed in plastic surgery in burnt individuals and also in cardiac bypass surgical procedures in which blood vessels from the legs are grafted to the heart region in the same individual. Low

 

Isograft Isograft is a type of grafting or transplantation in which tissues, cells or organs are grafted between individuals that are genetically identical (e.g. identical twins). This type of graft can also be called isogeneic graft; and it can also be used to transfer tissues between clones of an organism or inbred animals. Low
Allograft Allograft is a type of graft ion which tissues are grafted between members of the same species that are genetically different. This is the type of graft that is mostly obtainable in humans in which tissues, cells or organs are grafted from the donor host to the recipient host. Allograft can also be called homograft, and it is a transplant that occurs between allogenic individuals i.e. members of the same species who have different genetic makeup. High
Xenograft Xenograft is a tissue transplant in which cells or tissue is taken from an individual of one species and inserted or grafted into another individual of a different species (e.g. graft between man and monkey). This type of graft can also be called heterograft; and xenograft occurs in xenogenic individuals i.e. individuals of different species that have different genetic lineage. Rejection of grafted tissues or organs is greatest in xenografts because of the presence of high cross-reacting immunoglobulins from both the donor and recipients which induce a hyperacute rejection upon transplant. Very High
The greatest genetic discrepancy in tissue grafting is mostly experienced xenografts and this is due to the high level of genetic un-relatedness that exist between the donors and the recipient hosts. Autograft and isograft are often the most accepted transplantations and they experience little or no rejection in some cases. Allografts also express tissue rejection like the xenografts because of the dissimilarity that exist between the cells of the donor organism and that of the recipient host.

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