receptors specific for each invading organism. To account for all possible permutations of antigenic structure, natural and synthetic, that the host might encounter, the adaptive immune system uses genetic recombination of DNA and RNA splicing as a way of encoding its antibodies. Lymphocytes can recognize an estimated 107 different types of antigens through this genetic recombination mechanism, far more than a person is likely to encounter during a lifetime. Adaptive immunity is Ab-mediated immunity, based on circulating pools of antibodies that react with, and inactivate, antigens. These antibodies are found in the globulin fraction of the serum. Consequently, antibodies are also referred to as immunoglobulins (Ig). The adaptive immune response has the property of memory. The sensitivity, specificity, and memory for a particular antigen are retained, and subsequent exposures stimulate an enhanced response. Hence, the adaptive immune response differs from the innate in two respects: specificity and memory.
The adaptive immune response, like the innate, can be divided into two branches: humoral immunity and CMI. Humoral immunity is circulating immunity and is mediated by B lymphocytes and differentiated B lymphocytes known as plasma cells. CMI is controlled by the T lymphocytes. The immune function of T lymphocytes cannot be transferred by serum alone; the T cells must be present, whereas the immunity of the humoral system can be isolated from the serum and transferred. T cells are specially tailored to deal with intracellular infections (such as virus-infected cells), whereas B cells secrete soluble antibodies that can neutralize pathogens before their entry into host cells. Both B and T cells possess specific receptors on their surfaces to recognize unique stimulatory antigens. When B cells are stimulated, they express specific Ig or surface antibodies that are capable of binding to the antigen. A fraction of the B-cell population does not differentiate into Ab-producing cells but forms a pool of cells that retain the immunologi-cal memory. T cells express a specific antigen receptor, the T-cell receptor, similar in structure to the surface Ig receptor of B cells. This receptor is activated by a piece of processed antigen (presented with MHC-II). Activated T cells release soluble factors such as interleukins, cytokines, interferons, lymphokines, and colony-stimulating factors, all of which regulate the immune response. Interactions with some of these help to regulate the B-cell activity, directing the innate immune response.
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