Immunoglobulin M (IgM) is the largest antibody and the first to be produced in response to a new infection or antigen exposure. As a pentamer — five antibody units joined together — IgM is highly effective at activating the complement system and agglutinating pathogens during the early phases of infection, before IgG production has peaked.
Elevated IgM indicates acute or recent infection, as IgM levels peak within 1–2 weeks of exposure then decline as IgG takes over. Persistently elevated IgM is seen in Waldenström's macroglobulinaemia (a B cell cancer producing monoclonal IgM) and in certain autoimmune conditions. Low IgM is associated with selective IgM deficiency or combined immunodeficiency states.
FAQs
Why does elevated IgM indicate recent infection?
IgM is the first antibody produced in response to a new infection. It appears within days of exposure, peaks at 1-2 weeks, and then declines as IgG takes over. Elevated IgM-specific antibodies (e.g. IgM anti-HAV) indicate an acute or recent infection, while positive IgG alone indicates past exposure or vaccination.
What is Waldenstrom's macroglobulinaemia?
Waldenstrom's macroglobulinaemia is a B cell lymphoproliferative disorder that produces a monoclonal IgM paraprotein. It causes very high IgM, hyperviscosity of blood, anaemia, enlarged lymph nodes, and sometimes neuropathy or amyloid deposits. It is treated when symptomatic.
Can newborns have high IgM?
Elevated IgM in a newborn is significant because IgM does not cross the placenta - any IgM a newborn has produced itself in response to an infection. Elevated IgM in a neonate may indicate a congenital infection including toxoplasma, rubella, CMV, herpes, or syphilis.
Is isolated IgM deficiency serious?
Selective IgM deficiency can predispose to severe bacterial infections, particularly with encapsulated organisms. Severity varies widely and some individuals are largely asymptomatic while others have significant recurrent infection burden.
