IgM (Fc = μ) is present at a concentration ofabout 1.5 mg/mL and has a half-life of less than 1 week. ThisAb participates in opsonization, agglutination reactions, andcomplement fixation. Opsonization, as stated previously, is a“protein coating” or tagging of a bacterium that renders itmore susceptible to phagocytosis. A complex of the Fc portionof IgM plus C3b of complement is that protein. IgM isthe first immunoglobulin formed during immunization, but itwanes and gives way to IgG. IgM is a pentamer, and its agglutinationpotency is about 1,000 times that of IgG. IgM isalso responsible for the A, B, and O blood groups. The fundamentalmonomeric IgM structure is much like that of IgG.The pentamer is held together by disulfide bonds and a singleJ (joining) peptide. The affinity of an IgM monomer forantigen is less than that of IgG, but the multimeric structureraises the avidity of the molecule for an antigen.
