Medicine & USMLE

Immunoglobulin M (IgM)

  1. Immunoglobulin A (IgA)
  2. Immunoglobulin G (IgG)
  3. Immunoglobulin M (IgM)
  4. Immunoglobulin E (IgE)
  5. Immunoglobulin D (IgD)


Immunoglobulin M (IgM) is one of the five major subtypes of antibodies produced by B-cells. IgM is the default antibody type produced by B-cells, and does not require class switching to produce. As such, IgM is the first and fastest antibody to be made in response to a new antigen. IgM circulates as a pentamer, and the large size of the pentamer prevents IgM from crossing the placenta. However, this pentameric structure makes IgM an excellent fixer of complement, triggering the classical pathway of complement activation.

Key Points

  • Immunoglobulin M (IgM)
    • Archetypal/default antibody produced without class-switching 
      • IgM is expressed as an antigen receptor on surface of B-cells
      • Defects in class-switching lead to Hyper-IgM Syndrome
    • Produced immediately in response to new antigen
    • Circulates as a pentamer
      • Five IgM molecules joined by joining chain (J-chain)
    • Cannot cross placenta
      • large size of pentamer prevents passage
    • Fixes complement
    • Does not directly cause opsonization (unlike IgG)
      • Pentameric structure prevents Fc receptors from binding