Immunoglobulin M

IgM normally consists of 10 heavy μ-chains and 10 κ or λ type light chains which are always identical within a molecule. There is also a J chain linking all the μ chains together, so that simply speaking, IgM has a pentameric structure when compared to that of IgG. IgM is the largest immunoglobulin molecule (MW = 900000), but makes up only 6% of the plasma immunoglobulin. IgM is the first specific antibody to appear in the serum after infection. It is capable of activating complement, thus helping to kill bacteria. IgM levels sink after the infection has subsided at a relatively rapid rate compared to IgG. This fact is used to advantage in the differential diagnosis of acute and chronic infections by comparing specific IgM and IgG titers. If IgM is prevalent the infection is acute, whereas if IgG predominates the infection is chronic. Increased polyclonal IgM levels are found in viral, bacterial, and parasitic infections, liver diseases, rheumatoid arthritis, scleroderma, cystic Waldenstroem´s macroglobulinemia. Increased loss of IgM is found in protein-losing enteropathies and in burns. Decreased synthesis of IgM occurs in congenital and acquired immunodeficiency syndromes.