There are approximately 106-108 antibody specificities referring to different antigenic structures (epitopes), and a specific antigenic molecule will interact with and activate only a small proportion (<0.1%) of the antibody-producing cells. B cells are precursors of antibody-producing plasma cells, and T cells are thymus-derived cells that produce a number of stimulatory agents for B cells. The ability of a B-cell-derived immune (memory) cell to produce antibodies depends on a number of stimulatory factors as well as transpositions of variable genetic elements of the immunoglobulin structure, a subject of intense concentration in molecular genetics. Stimulatory activities are derived from thymus-derived cells (T cells) usually considered to be helper T cells (Figure 17-1).
The thymus contains T-cell lymphocytes, which in turn produce factors that regulate the expression of immunoglobulins by B-cell lymphocytes. T cells enter circulation from the thymus, as do B cells from bone marrow. Phagocytic cells (macrophage, antigen-processing cell, APC) are also involved in the immune process.
When an antigen (a foreign substance) enters the organism, B cells produce immunoglobulins with helper T-cell action. Antibodies on the surface of T cells with the same specificity as those produced by the activated B cells bind the antigen, and the complex is then transferred to a macrophage cell that ingests and destroys the antigen. This scheme (Figure 17-1) also depicts the factors contributed by helper T cells that stimulate antibody production. This chapter concentrates on a family of peptide hormones that regulate the proliferation and maturation of precursor lymphocytes into immunologically competent cells.
When an antigen is recognized by a T cell, a signal transduction mechanism involving the T-cell receptor comes into play. This series of events is shown in Figure 17-2. This figure shows the signal transduction events
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