Distinct cellular events are induced by antigen-mediated cross-linking of the BCR by different types of antigens: multivalent antigens initiate B cell proliferation and differentiation, and protein antigens prepare B cells for subsequent interactions with helper T cells. Antigen receptor cross-linking by some antigens can stimulate several important changes in B cells (Fig. 11-6). The previously resting cells enter into the Gj stage of the cell cycle, and this is accompanied by increases in cell size, cytoplasmic RNA, and biosynthetic organelles such as ribosomes The survival of the stimulated B cells is enhanced as a result of the production of various anti-apoptotic proteins, notably Bcl-2 (see Fig. 14-7, Chapter 14), and the cells may proliferate and secrete some antibody. Activation of B cells by antigen results in increased expression of class II major histocompatibility complex (MHC) molecules and B7 costimulators, because of which antigen-stimulated B cells are more efficient activators of helper T lymphocytes than are naive B cells. The expression of receptors for several T cell-derived cytokines is also increased, which enables antigen-stimulated B lymphocytes to respond to cytokines secreted by helperT cells. The expression of chemokine receptors may change, resulting in movement of the B cells out of the follicles.
The importance of signaling by the BCR complex for the subsequent responses of the cells may vary with the nature of the antigen. Most T-independent antigens, such as polysaccharides, display multiple identical epi-topes on each molecule or on a cell surface. Therefore, such multivalent antigens effectively cross-link many B cell antigen receptors and initiate responses even though they are not recognized by helper T lymphocytes. In contrast, many naturally occurring globular protein antigens possess only one copy of each epitope per molecule. Therefore, such protein antigens cannot simultaneously bind to and cross-link multiple Ig molecules, and their ability to activate the BCR is limited, so they do not typically induce signals that can drive B cell proliferation and differentiation. However, some protein antigens may be displayed as multivalent arrays on the surfaces of microbes or cells, or they may be multivalent because they are in aggregates. Protein antigens are also internalized by the BCR and processed and displayed to helper T cells, which are potent stimulators of B lymphocyte proliferation and differentiation. In fact, in T-dependent responses, a major function of membrane Ig may be not signaling but binding and internalization of the antigen for subsequent presentation to helper T cells. Therefore, protein antigens may only activate the BCR to enhance antigen presentation and the migration of antigen-specific B cells toward the T cell zone.
After specific B cells recognize antigens, the subsequent steps in humoral immune responses are very different in T-dependent and T-independent responses. We next describe the activation of B cells by protein antigens and helper T cells.
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