Immunoglobulin M is the first isotype to be expressed by the adaptive immune system in response to a foreign pathogen, during B cell development. It is typically present as the antigen receptor on the naïve B-cell surface, allow for the B cell to associate with the polypeptides CD79a and CD79b, which play roles in IgM cell signaling.
IgM is monomeric when first produced by plasma cells but when secreted can link to other IgM units to form a pentameric configuration containing a polypeptide J-chain. This occurs during maturation and antigenic stimulation and increases IgM’s avidity since there would be ten antigen-binding sites per pentameric molecule. Through interactions with the antigen, IgM can then opsonize it for destruction.
The detection of IgM can be used to diagnose patients from acute exposure to a pathogen, since IgM is typically linked with a primary immune response. However, it does not indicate whether the patient still has that pathogen or if it had been eliminated, since memory plasma cells help form immunological memory to generate antibodies against a pathogen for many more months and years to come.1
In addition, several disorders can be connected with IgM, such as X-linked Hyper-IgM Syndrome. This is a rare primary immune deficiency disorder, characterized by elevated IgM levels but deficient levels of other immunoglobulin isotypes, as well as defects in cellular immunity. Other disorders linked to IgM include Selective IgM Deficiency, Cold Agglutinin Disease, and Monoclonal Gammopathies.2
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References:
Schroeder, H. W., & Cavacini, L. (2010). Structure and Function of Immunoglobulins. The Journal of Allergy and Clinical Immunology, 125(2 0 2), S41. https://doi.org/10.1016/J.JACI.2009.09.046
