Non-clinical sciences/Immunology · Immunoglobulins

IgM

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What it means

First antibody produced in the PRIMARY immune response. Circulates as a pentamer (10 antigen-binding sites) → strong agglutination and a potent classical-complement activator. Too large to cross the placenta. The ABO isohaemagglutinins (anti-A/anti-B) are IgM.

Worked example

Antigen-specific IgM positivity = recent/acute infection (e.g. anti-HBc IgM in the hepatitis-B window; toxoplasma/CMV IgM).

In the exam

Serology in a patient with a recent febrile illness shows a large, pentameric antibody that fixes complement avidly but cannot reach the fetus across the placenta.

What settles it

IgM = pentamer, acute marker, no placental transfer — vs IgG (monomer, most abundant, crosses placenta, late/memory marker).

Classically confused with

IgG

Source: StatPearls — Immunoglobulin