Immunoglobulin M

Immunoglobulin M, or IgM for short, is a basic antibody that is produced by B cells. IgM is by far the physically largest antibody in the human circulatory system. It is the first antibody to appear in response to initial exposure to an antigen. The spleen, where plasmablasts responsible for antibody production reside, is the major site of specific IgM production.

IgM is a polymer, where multiple immunoglobulins are linked together by strong covalent bonds known as disulfide bonds. This occurs mostly to produce pentamers (5 linked immunoglobulins) but also as a hexamer (six linked immunoglobulins). IgM has a molecular mass of approximately 970 kDa (in its pentamer form). Because each immunoglobulin monomer has two antigen binding sites, a pentameric IgM has 10 binding sites. Typically, however, IgM cannot bind 10 antigens at the same time because the large size of most antigens hinders binding to nearby sites.

The J chain is found in pentameric IgM but not in the hexameric form, perhaps due to space constraints in the hexameric complex. Pentameric IgM can also be made in the absence of J chain. At present, it is still uncertain what fraction of normal pentamer contains J chain, and to this extent it is also uncertain whether a J chain-containing pentamer contains one or more than one J chain. Although hexameric IgM without J chain has higher efficiency of complement fixation than pentameric IgM with J chain.

Because IgM is a large molecule, it cannot diffuse well, and is found in the interstitium only in very low quantities. IgM is primarily found in serum; however, because of the J chain, it is also important as a secretory immunoglobulin.

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