IgM

Immunoglobulin M (IgM) is one of the major classes of antibodies produced by the immune system. In adults, serum IgM typically circulates as a pentamer, composed of five basic antibody units linked by a J chain. Each unit contains two light chains and two mu heavy chains. IgM also exists as a membrane-bound form on naive B cells, where it functions as part of the B cell receptor in conjunction with IgD.

IgM plays a crucial role in the early stages of the immune response. It is the first antibody isotype produced during a primary infection and has high overall avidity because of its pentameric structure, even though individual binding sites have relatively low affinity. This makes IgM highly effective at agglutination of pathogens and at initiating the classical complement pathway by binding C1q. Consequently, IgM is particularly important in defense against extracellular bacteria and newly encountered antigens, including many encapsulated organisms. There is also a population of natural IgM antibodies produced without prior antigen exposure, contributing to immediate, low-affinity protection.

In terms of distribution and clinical relevance, IgM constitutes a relatively small portion of total serum immunoglobulin, with concentrations generally around 0.5 to 2 mg/mL, and it has a shorter serum half-life than IgG (roughly several days). IgM does not cross the placenta, so maternal IgM does not provide fetal immunity; detection of IgM antibodies in a newborn can indicate a recent or active infection. Serology often uses IgM to indicate acute infection (for example, IgM against a virus). Pathologically, IgM can be implicated in certain disorders, such as rheumatoid factor activity (an IgM autoantibody against IgG Fc) and IgM-related gammopathies like Waldenström’s macroglobulinemia, which can cause hyperviscosity.