Immunoglobulin M, Quantitative

Immunoglobulin M is a pentamer of 7S γ-globulin and is an efficient complement binder. It is the antibody type produced initially in the immune response and the first immunoglobulin class to be synthesized by a fetus or newborn. IgM antibodies do not cross the placenta. For these reasons the demonstration of IgM-specific antibody is useful in assessing whether a particular infection is acute (in which case IgM antibodies will be present) or chronic (IgG antibodies will predominate) and whether a newborn has a congenital infection (a newborn with IgM antibody is infected; a newborn with IgG antibody has passively acquired maternal antibody, which simply crossed the placenta). In the hyper-IgM immunodeficiency syndrome, there is an absence of IgG and IgA in serum and a marked increase in IgM. Macroglobulins produced in Waldenström disease are IgM, and may produce hyperviscosity syndrome. More than 2 g/dL of monoclonal IgM is a major diagnostic criterion of myeloma. Increased IgM (with other immunoglobulins) may develop in inflammatory/infectious conditions. IgM is characteristically elevated in primary biliary cirrhosis. The majority of rheumatoid factors are IgM. IgM will be decreased in congenital or acquired hypogammaglobulinemia, and this will be associated with increased, recurrent infection.