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Background
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Immunoglobulin M (IgM) forms polymers where multiple immunoglobulins are covalently linked together with disulfide bonds, normally as a pentamer and the J chain is attached to most pentamers. It has a large molecular mass of approximately 900 kD. Due to its polymeric nature, IgM possesses high avidity, and is particularly effective at complement activation. It is sometimes called a "natural antibody", but it is likely that the antibodies arise due to sensitization in the very young to antigens that are naturally occurring in nature. IgM is the first immunoglobulin expressed by mature B cells. IgM antibodies appear early in the course of an infection and usually do not reappear after further exposure. IgM antibodies do not pass across the human placenta. These two biological properties of IgM make it useful in the diagnosis of infectious diseases. Demonstrating IgM antibodies in patients serum indicates recent or in serum from a neonate intrauterine infection such as congenital rubella.
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