B lymphocytes

bursa-dependent lymphocytes in birds and their counterparts in nonavian vertebrates including human beings, the cells primarily responsible for humoral immunity, the precursors of antibody-producing cells (plasma cells). In birds maturation of these cells takes place in the bursa of Fabricius; the hypothesized analogous tissue in other vertebrates was termed the “bursa-equivalent” tissue. It now appears that B lymphocyte maturation occurs primarily in the bone marrow in mammals. B lymphocytes are characterized by the presence of surface immunoglobulin, monomeric IgM or IgD, which constitutes the B-cell antigen receptors. When stimulated by antigen, a process that requires the cooperation of helper T cells and macrophages, B lymphocytes proliferate and differentiate into plasma cells and memory B cells. The entire clone of cells descended from a single activated B lymphocyte produces immunoglobulins having the same antigen combining site as that in the antigen receptors of the original cell; thus all of the antibody produced and all of the memory cells are specific for the antigen that induced their formation. Called also B cells.