B cell

B cells, or B lymphocytes, are white blood cells responsible for antibody-based, or humoral, immunity. They are made in the bone marrow from blood-forming stem cells, and there each one builds a unique B cell receptor, a membrane-bound antibody, and cells that react strongly to the body's own molecules are removed. Mature B cells then circulate through the blood and lymphoid organs such as lymph nodes and the spleen.

Unlike T cells, a B cell can recognize an antigen in its natural, unprocessed form. For most protein antigens, it takes the antigen in, displays fragments on MHC class II molecules and needs signals from a helper T cell to become fully active. The selected cell then divides into a clone. Many of its offspring become plasma cells that secrete large amounts of antibody with the same specificity, often switching from IgM to IgG, IgA or IgE. Others become long-lived memory B cells, which give a faster, stronger response on re-exposure.

The letter B comes from the bursa of Fabricius, the organ in birds where these cells' role in antibody production was discovered; in mammals they mature in bone marrow, a handy coincidence.