Some of the progeny of the B cells that have proliferated in response to antigen and T cell help differentiate into antibody-secreting plasma cells. Plasma cells are morphologically distinct, terminally differentiated B cells committed to abundant antibody production (see Chapter 2). They are generated after the activation of B cells through signals from the BCR, CD40, TLRs, and other receptors including cytokine receptors. There are two types of plasma cells. Short-lived plasma cells are generated during T-independent responses and early during T cell-dependent responses in extrafollicular B cell foci, described earlier. These cells are generally found in secondary lym-phoid organs and in peripheral nonlymphoid tissues. Long-lived plasma cells are generated in T-dependent germinal center responses to protein antigens. Signals from the B cell antigen receptor and IL-21 cooperate in the generation of plasma cells. Plasma cells (and their precursors, plasmablasts, which may be found in the circulation) generated in germinal centers acquire the ability to home to the bone marrow, where they are maintained by cytokines of the BAFF family which bind to a plasma cell membrane receptor called BCMA, thus allowing the plasma cells to survive for long periods, often as long as the life span of the host. Typically 2 to 3 weeks after immunization with a T cell-dependent antigen, the bone marrow becomes a major site of antibody production. Plasma cells in the bone marrow may continue to secrete antibodies for months or even years after the antigen is no longer present. These antibodies can provide immediate protection if the antigen is encountered later. It is estimated that almost half the antibody in the blood of a healthy adult is produced by long-lived plasma cells and is specific for antigens that were encountered in the past. Secreted antibodies enter the circulation and mucosal secretions, but mature plasma cells do not recirculate.
The differentiation of activated B cells into antibody-secreting plasma cells involves major structural alterations, especially of components of the endoplasmic reticulum and secretory pathway, and increased Ig production as well as a change in Ig heavy chains from the membrane to the secreted form. Although a plasma cell is derived from a B cell, it undergoes a remarkable transformation as it differentiates from an activated B cell to the plasmablast stage. Most strikingly, the cell enlarges dramatically, and the ratio of cytoplasm to nucleus also undergoes a striking increase. The endoplasmic reticulum becomes prominent, and the cell is transformed into a secretory cell that bears little or no resemblance to a B cell. The transcription factors that regulate the development of plasma cells are described later.
The change in Ig production from the membrane form (characteristic of B cells) to the secreted form (in plasma cells) is the result of changes in the carboxyl terminal of the Ig heavy chain (Fig. 11-20). For instance, in
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