Immunoglobulin M (IgM)

Immunoturbidimetry assay

Product description:

Immunoglobulin M (IgM) are the first immunoglobulins synthesized by neonates and are the preponderant class of immunoglobulin molecules appearing during early phases of immune responses. IgM is a high molecular weight protein (macroglobulin), consisting of five or rarely of six subunits (IgM monomers). The IgM monomers are found at a low concentration in human serum. Each pentameric IgM molecule is composed of 10 heavy (?) chains, 10 light chains and usually one joining (J) chain. The B cells can also secrete functionally active IgM hexamers lacking J chains but the amount of hexamers in serum is no more than 5% of total IgM. A pentameric IgM molecule has 10-antigen combining sites and can bind 10 small antigens (haptens). However, due to steric restrictions, only five large antigen molecules can be bound by one IgM molecule. The antibody activity of IgM is destroyed upon reduction of the inter-subunit disulfide linkages.

In the monomeric form, IgM functions as an antigen-specific part of the B-cell antigen receptor on the surface of unstimulated B lymphocytes. The polymeric IgM molecules are very efficient activators of the classical complement cascade. A single IgM molecule can activate complement component C1, whereas for that several IgG molecules are needed. The hexamer IgM molecules are much more efficient in activation of complement than pentameric IgM but they have nearly the same avidity to antigen as pentamers. Due to the multiplicity of their combining sites, both pentamer and hexameric forms of IgM are very efficient in agglutination and cytolytic reactions.

IgM concentration is decreased in diseases related with hereditary or acquired deficiencies of the immunoglobulin production. Polyclonal increases in serum immunoglobulins are the normal response to infections. The IgM generally increases as a primary response to virus infections and blood stream infections such as malaria and primary biliary cirrhosis.