IgM consists of 10 heavy µ-chains and 10 kapa chains or lambda 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= 900,000), but makes up only 6% of the plasma immunoglobulins.
IgM is the first specific antibody to appear after infection. It is capable of activating complement, thus helping to kill bacteria. IgM levels sink after infection has subsided at a rapid rate compared to IgG. This fact is used in differential diagnosis of acute and chronic infections by comparing specific IgM and IgG titers. If IgM is prevalent than 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 fibrosis and heroin addiction.
Monoclonal IgM is increased in Waldenstrom’s macroglobulinemia. Increased loss of IgM is found in protein-losing enteropathies and in burns. Decreased synthesis of Igm occurs in congenital and aquired immunodeficiencies.
Quantitation of IgM can be done by turbidometry and nephelometry techniques.