he developmental process that results in production of plasma cells and memory B cells can be divided into three broad stages: generation of mature, immunocompetent B cells (maturation), activation of mature B cells when they interact with antigen, and differentiation of activated B cells into plasma cells and memory B cells. In many vertebrates, including humans and mice, the bone marrow generates B cells. This process is an orderly sequence of Ig-gene rearrangements, which progresses in the absence of antigen. This is the antigen-independent phase of B-cell development.
A mature B cell leaves the bone marrow expressing membrane-bound immunoglobulin (mIgM and mIgD) with a single antigenic specificity. These naive B cells, which have not encountered antigen, circulate in the blood and lymph and are carried to the secondary lymphoid organs, most notably the spleen and lymph nodes (see Chapter 2). If a B cell is activated by the antigen specific to its membrane-bound antibody, the cell proliferates (clonal expansion) and differentiates to generate a population of antibody-secreting plasma cells and memory B cells. In this activation stage, affinity maturation is the progressive increase in the average affinity of the antibodies produced and class switching is the change in the isotype of the antibody produced by the B cell from ^ to 7, a, or e. Since B cell activation and differentiation in the periphery require antigen, this stage comprises the antigen-dependent phase of B-cell development.
Many B cells are produced in the bone marrow throughout life, but very few of these cells mature. In mice, the size of the recirculating pool of B cells is about 2 X 108 cells. Most of these cells circulate as naive B cells, which have short life spans (half lives of less than 3 days to about 8 weeks) if they fail to encounter antigen or lose in the competition with other B cells for residence in a supportive lymphoid environment. Given that the immune system is able to generate a total antibody diversity that exceeds 109, clearly only a small fraction of this potential repertoire is displayed at any time by membrane immunoglobulin on recirculating B cells. Indeed, throughout the life span of an animal, only a small fraction of the possible antibody diversity is ever generated.
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