immunoglobulin M (IgM), one of the five classes of antibodies produced by the body and the largest in molecular structure. It is found in circulating fluids and is the first immunoglobulin to appear on the cell surface of B cells and the first to be produced when the body is challenged by antigens. IgM triggers the increased production of immunoglobulin G and the complement fixation required for effective immune responses. It is the dominant antibody in ABO blood group incompatibilities. The normal concentration of IgM in serum is 40 to 120 mg/dL. Compare immunoglobulin A, immunoglobulin D, immunoglobulin E, immunoglobulin G.
