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.
Was this article helpful?
An Open Letter To Anyone Who Wants To Lose Up To 20 Pounds In 30 Days The 'Low Carb' Way. 30-Day Low Carb Diet 'Ketosis Plan' has already helped scores of people lose their excess pounds and inches faster and easier than they ever thought possible. Why not find out what 30-Day Low Carb Diet 'Ketosis Plan' can do for you by trying it out for yourself.