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IgM is the largest immunoglobulin molecule (with a pentameric structure), but makes up only 6 % of the plasma immunoglobulins.
IgM is the first specific antibody to appear in the serum after infection. It is capable of activating complement, thus helping to kill bacteria. After the infection has subsided, IgM levels sink 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 (e.g. rubella, viral hepatitis).
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 Waldenström’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. Due to the slow onset of IgM synthesis, the IgM concentration in serum from infants is lower than in that from adults.
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