A protein (see immunoglobulin) produced by certain white blood cells (plasma cells) in response to entry into the body of a foreign substance (antigen) in order to render it harmless. An antibody-antigen reaction is highly specific. Antibody production is one aspect of the immune response and is stimulated by such antigens as invading bacteria, foreign red blood cells (see ABO system), inhaled pollen grains or dust, and foreign tissue grafts. Specific monoclonal antibodies are now used in various types of immunoassay. See also B cell; immunity.