Antibody mediated immunity

Antigen-specific activation of B cells occurs following the binding of antigen to membrane-bound immunoglobulin. Under the influences of a variety of cytokines released from monocytes and T cells (see Development of the immune system, section II.B.), B cells undergo clonal expansion and finally, differentiation into plasma cells capable of secreting large quantities of antibody. Small subsets of mature B cells become memory B cells, which are responsible for the recall responses after reexposure to antigen.

When an individual first encounters a foreign antigen, an antibody response is mounted. There are typically four stages that characterize the primary immune response. During the first stage, no antibody is detected for the first 4 to 5 days. During the second phase, IgM antibodies form in high titers, followed several days later (typically 10 to 14 days after antigen exposure) by the production of IgG antibodies directed toward the same antigen. In the third stage, the antibody titer stabilizes, and during the fourth stage, there is a decline in antibody titer over a period of months to years, as the antibody is either cleared or catabolized. The secondary antibody response occurs upon reexposure to the same antigen; antibody, primarily IgG, appears more rapidly, persists longer, and reaches a higher titer.