B cell (B lymphocyte) A lymphocyte that is derived from stem cells in the bone marrow but does not mature in the thymus (compare T cell); in birds it matures in the bursa of the cloaca (hence B cell). Each B cell has a unique set of receptor molecules on its surface, designed to recognize a specific antigen. Binding of the appropriate antigen to class II MHC receptors on the B cell (see histocompatibility) is recognized by helper T cells, which themselves bind to the antigen–receptor complex. This triggers the T cells to release lymphokines, which cause the B cell to undergo repeated division to form a clone of cells (i.e. clonal expansion). These mature into plasma cells, capable of producing large amounts of specific antibody (see immunoglobulin), which circulates in the blood and lymph and binds to the corresponding antigen. After a few days of antibody production the plasma cells die. However, some cells from the clone remain in the form of memory cells, which initiate a more rapid immune response on subsequent exposure to the same antigen. See also clonal selection theory.
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