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| Clonal Selection |
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| B and T lymphocytes have specific receptors on their surface. In B cells, this receptor is the antibody produced by that cell. When this receptor interacts with the antigenic determinant specific to it, the B lymphocyte becomes activated and divides to form a clone of cells. These cells also gets transformed into antigen producing B-cells and T cytotoxic cells. This phenomenon is called clonal selection, because all the cells in a given T or B cells clone are derived from a single parental cell. They exhibit the same specificity for antigenic determinant. |
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| But some activated lymphocytes develop into memory cells instead and donot produce antibodies immediately. They have a long life. |
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| The immune response due to first encounter with an antigen takes a longer time, feeble and declines rapidly. After the initial contact with an antigen, there is a period of several days during which no antibody is present, then there is a slow rise in the antibodies, followed by a gradual decline. Such a response of the body to the first contact with an antigen is called the Primary response. |
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| But when the antigen is contacted again, there is an immediate proliferation of lymphocytes and the antibodies are produced in far greater numbers. This accelerated and more intense response is called the Secondary response. |
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| This is due to the memory cells that were produced during the primary response, which lasts for a long time, sometimes even for a life-time. This is the reason why a person who had once suffered from chicken pox or measles becomes immune to subsequent attacks of the same disease. |
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| Lymphoid organs |
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| These are organs where maturation and proliferation of lymphocytes takes place. Lymphoid organs where T and B lymphocytes mature and acquire their antigen - specific receptors are called Primary Lymphoid organs. Bone marrow (site of B cell maturation) and Thymus gland (site of T-cell maturation) are primary lymphoid organs. After maturation, B and T cells travel through circulatory system to organs called secondary lymphoid organs. They are lymph nodes, spleen and tonsils. These organs are the sites for proliferation and differentiation of lymphocytes, in response to specific antigens. |
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