When antigen binds to a B-cell receptor, that B cell becomes poised to respond. In most cases, however, the B cell requires confirmation by an effector T-helper cell that the antigen truly merits a response. Only when this occurs can the B cell begin dividing, differentiating, and, finally, producing antibodies. Compounds that evoke a response by B cells only with the assistance of effector T-helper cells are called T-dependent antigens; these antigens are generally proteins. Antigens to which B cells can respond without the aid of effector T-helper cells are called T-independent antigens; they are generally carbohydrates and lipids.
We will begin by describing the role of effector T-helper cells in B cell activation. Later in the chapter, we will explain how naive T-cells become activated to attain those effector functions.
When a T-dependent antigen binds to a B-cell receptor, the B cell internalizes the antigen, enclosing it within a membrane-bound vacuole inside the B cell. Within that vacuole the antigen is degraded into peptide fragments that are delivered to glyco-proteins called MHC class II molecules that then move to the B-cell surface (figure 16.9). This process, called antigen presentation, "presents" pieces of the antigen for inspection by effector T-helper cells. Recall that T cells have on their surface multiple copies of an antigen-specific receptor called a T-cell receptor, which is functionally analogous to a B-cell receptor. If the receptor of an effector T-helper cell binds to one of the pep-tide fragments being presented by the cell, then that T cell activates the B cell. It does this by delivering cytokines to the B cell, initiating the process of clonal expansion of that cell. If the population of effector T-helper cells fails to recognize any of the fragments being presented by the B cell, then that B cell may become unresponsive to future exposure to the antigen. This induces tolerance to that antigen, endowing the adaptive immune system with a mechanism to avoid erroneous responses against "self" antigens. ■ tolerance, p. 394 ■ glycoprotein, p. 29
As activated B cells continue proliferating, some of the resulting progeny differentiate to form plasma cells, which may be viewed as effector B cells (figure 16.10). These highly specialized antibody-producing cells are capable of synthesizing thousands of molecules of antibody per second. After the cells produce quantities of antibodies, they die.
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