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.
Abbas A.K, Lichtman A.H and Pillai S (2010). Cellular and Molecular Immunology. Sixth edition. Saunders Elsevier Inc, USA.
Actor J (2014). Introductory Immunology. First edition. Academic Press, USA.
Alberts B, Bray D, Johnson A, Lewis J, Raff M, Roberts K and Walter P (1998). Essential Cell Biology: An Introduction to the Molecular Biology of the Cell. Third edition. Garland Publishing Inc., New York.
Bach F and Sachs D (1987). Transplantation immunology. N. Engl. J. Med. 317(8):402-409.
Barrett J.T (1998). Microbiology and Immunology Concepts. Philadelphia, PA: Lippincott-Raven Publishers. USA.
Jaypal V (2007). Fundamentals of Medical Immunology. First edition. Jaypee Brothers Medical Publishers (P) Ltd, New Delhi, India.
John T.J and Samuel R (2000). Herd Immunity and Herd Effect: New Insights and Definitions. European Journal of Epidemiology, 16:601-606.
Levinson W (2010). Review of Medical Microbiology and Immunology. Twelfth edition. The McGraw-Hill Companies, USA.
Roitt I, Brostoff J and Male D (2001). Immunology. Sixth edition. Harcourt Publishers Limited, Spain.
Zon LI (1995). Developmental biology of hematopoiesis. Blood, 86(8): 2876–91.