A patient's serum is tested for IgM antibodies against a specific pathogen. Which structural feature of IgM makes it highly effective at agglutination and complement activation despite typically being present at a lower serum concentration than IgG?
- A It has a long hinge region
- B It is a pentamer with 10 antigen-binding sites ✓
- C It is a monomer with high affinity
- D It crosses the placenta easily
Correct answer: B. It is a pentamer with 10 antigen-binding sites
Explanation
IgM is secreted as a pentamer held together by a J chain. This pentameric structure gives IgM 10 antigen-binding sites, making it extremely efficient at agglutination and complement activation despite its lower serum concentration compared to IgG. IgG is a monomer and crosses the placenta.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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