The antibody-mediated or humoral immunity is that where the B-lymphocytes synthesize antibodies response to the detection of antigens and these antibodies counteract with those antigens. Antibody-mediated immunity is often referred to as humoral immunity because the antibody molecules flow extracellularly through the blood and other vital body fluids which are called humors in Greek.
The precursors of B-lymphocytes, which originate from the stem cells of bone marrow, processed within thymus-independent tissues of the lymphatic system (the spleen, tonsils, intestine, appendix, and lymph nodes) and become immunologically competent. In response to antigenic stimulation, the immunologically competent B-lymphocytes convert into two different cell populations primary B-lymphocytes and secondary B- lymphocytes.
The primary B-lymphocytes show response to the first antigenic stimulation and immediately enter into the process of their conversion into plasma cells. In contrast, the secondary B-lymphocytes do not show any response to the first antigenic stimulation and constitute memory cells which transform into plasma cells in response to subsequent exposure to antigens, perhaps years later. However, these are the plasma cells which secrete antibodies.
Once thought to be totally independent system, the conversion of immunologically competent B- lymphocytes into antibody producing plasma cells is cooperated by helper T-lymphocytes (helper T-cells). An antigen with at-least two different antigenic determinants first attaches on the surface of a macrophage cell which then migrates to lymphatic tissues and interact with helper T-lymphocyte.
Once the helper T-lymphocyte binds with the antigen present on the surface of microphage cell, it releases interleukin-2 that stimulates multiplication of helper T-lymphocytes and also the release of B-lymphocyte growth factor, which in turn enhances the division of immunologically competent B-lymphocytes and their conversion into plasma cells.
However, the antibodies secreted by plasma cells clump together (agglutinate) with antigens present in body’s circulatory system forming antibody-antigen-complexes which are up-taken by scavenger white blood cells. The diagrammatic representation of the process of antibody-mediated (humoral) immunity is given in Fig. 41.3.