| B cells enter the bloodstream. When they encounter antigens, they mature into B plasma cells, which secrete antibodies that confer humoral immunity to the antigen.
They attack by binding host cells and lysing them (cellular immunity).
21.14 Immune responses are divided into humoral immunity, in which antibodies are produced by B cells, and cellular immunity produced by T cells.
one and only one specific antigen: each mature B cell produces antibodies against a single antigen, and each T cell is capable of attaching to only one type of foreign antigen.
If each lymphocyte is specific for only one type of antigen, how does an immune response develop? The theory of clonal selection proposes that initially there is a large pool of millions of different lymphocytes, each capable of binding only one antigen (I Figure 21.15); so millions of different foreign antigens can be detected. To illustrate clonal selection, let's imagine that a foreign protein enters the body. Only a few lymphocytes in the pool will be specific for this particular foreign antigen. When one of these lymphocytes encounters the foreign antigen and binds to it, that lymphocyte is stimulated to divide. The lymphocyte proliferates rapidly, producing a large population of genetically identical cells—a clone—each of which is specific for that particular antigen.
This initial proliferation of antigen-specific B and T cells is known as a primary immune response (see Figure 21.15); in most cases, the primary response destroys the foreign antigen. Subsequent to the primary immune response, most of the lymphocytes in the clone die, but a few continue to circulate in the body. These memory cells may remain in circulation for years or even for the rest of one's life. Should the same antigen reappear at some time in the future, memory cells specific to that antigen become activated and quickly give rise to another clone of cells capable of binding the antigen. The rise of this second clone is termed a secondary immune response (see Figure 21.15). The ability to quickly produce a second clone of antigen-specific cells permits the long-lasting immunity that often follows recovery from a disease. For example, people who have chicken pox usually have life-long immunity to the disease. The secondary immune response is also the basis for vaccination, which stimulates a primary immune response to an antigen and results in memory cells that can quickly produce a secondary response if that same antigen appears in the future.
Three sets of proteins are required for immune responses: antibodies, T-cell receptors, and the major histocompatibility antigens. The next section explores how the enormous diversity in these proteins is generated.
Each B cell and T cell of the immune system is genetically capable of binding one type of foreign antigen. When a lymphocyte binds to an antigen, the lymphocyte undergoes repeated division, giving rise to a clone of genetically identical lymphocytes (the primary response), all of which are specific for that same antigen. Memory cells remain in circulation for long periods of time; if the antigen reappears, the memory cells undergo rapid proliferation and generate a secondary immune response.
The principal products of the humoral immune response are antibodies—also called immunoglobulins. Each immunoglobulin (Ig) molecule consists of four polypeptide chains—two identical light chains and two identical heavy chains—which form a Y-shaped structure (IFigure 21.16). Disulfide bonds link the two heavy chains in the stem of the Y
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