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