Immunoglobulin M, Plasma; Purified

Immunoglobulin M (IgM) is the third most common serum immunoglobulin, behind immunoglobulin G (IgG) then immunoglobulin A (IgA). IgM is the first immunoglobulin produced by a fetus and is the first produced by naive B cells after stimulation by an antigen.

Structurally, IgM usually exists as a pentamer, but it can also be present as a monomer. In the pentameric form, all heavy chains and light chains are respectively identical. A protein called the J chain is covalently bound to IgM via a dislufide bridge and serves to polymerize the molecule into a pentamer. (The monomer does  not contain the J chain protein.)

Due to its pentameric structure, IgM is excellent at complement fixation and agglutination. As such, IgM antibodies are good at capturing microorganisms for elimination from the body. IgM is also present on the surface of B cells, functioning as a receptor for antigens. In this capacity, IgM associates noncovalently with two proteins in the B cell membrane, Ig-alpha and Ig-beta. These proteins facilitate intracellular signaling after IgM binds to an antigen, as the IgM tail is too short to reach the cytoplasmic side of the B cell membrane.

Increases in IgM may be indicative of Waldenstrom's macroglobulinemia, Carrion's disease, infectious mononucleosis, malaria, lupus erythematosus, and rheumatoid arthritis, among other disorders. Low levels of IgM may be found in individuals with such afflictions as agammaglobulinemia, some cases of lymphoproliferative disorders, lymphoid aplasia, IgG and IgA myeloma, chronic lymphoblastic leukemia, and more.