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The immune system of vertebrates produces a collection of five different immunoglobulins. Immunoglobulin M, known as IgM, is one of them. The name IgM derives from the fact that the heavy chain of IgM is the largest and is called µ. In addition, although produced as monomers by B cells, IgM antibodies are secreted as a pentamer (5 covalently linked antibodies). Interestingly, IgM antibodies are one of the first antibodies that appear in the blood of the human fetus.

Due to its pentameric structure, an IgM antibody can have high avidity for antigen (a combination of the affinity of individual antigen binding sites to antigen epitopes and the valency of the antibody). Unlike IgM, which are divalent, IgM are pentavalent and can, therefore, have a higher avidity despite a low antigen-antibody affinity constants.

Human serum contains several antibodies of type IgM. Rheumatoid factor(RF) (an auto-antibody that targets the Fc region of various IgG) is often of IgM type. Because many immuno-assays use IgG antibody in analyte detection, the IgM nature of RF can be a major source of interference, leading to inaccurate results, false positives or false negatives.

Counteracting the effect of RF is essential when developing an accurate immuno-assay. One way to counteract the effect of RF is to add a large excess of antibodies that target specifically IgM antibodies. As such, adding anti-human IgM antibodies to immuno-assay reagents is often required to counteract the effect of RF.

Using a collection of several anti-IgM antibodies can be very effective at blocking RF interferference (see Rheumatoid Factor Interference Blocker).