Plasma cells, also called plasma B cells, plasmocytes, and effector B cells, are white blood cells that secrete large volumes of antibodies. They are transported by the blood plasma and the lymphatic system. Like all blood cells, plasma cells originate in the bone marrow; however, these cells leave the bone marrow as B cells, before terminal differentiation into plasma cells, normally in lymph nodes.
After leaving the bone marrow, the B cell acts as an antigen presenting cell (APC) and internalizes offending antigens, which are taken up by the B cell through receptor-mediated endocytosis and processed. Pieces of the antigen (which are now known as antigenic peptides) are loaded onto MHC II molecules, and presented on its extracellular surface to CD4 T cells (sometimes called T helper cells). These T cells bind to the MHC II-antigen molecule and cause activation of the B cell.
Upon stimulation by a T cell, which usually occurs in germinal centers of secondary lymphoid organs like the spleen and lymph nodes, the activated B cell begins to differentiate into more specialized cells. Germinal center B cells may differentiate into memory B cells or plasma cells. The mechanism by which a B cell becomes one or the other of these is a process known as affinity maturation. Most of these B cells will become plasmablasts (or "immature plasma cells"), and eventually plasma cells, and begin producing large volumes of antibodies.
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