A 58-year-old woman presents with fatigue, pallor, and acrocyanosis on cold exposure. Hemoglobin is 9.2 g/dL with reticulocytosis. Direct Coombs test is positive for C3d but negative for IgG. Cold agglutinin titer is elevated. Which immunoglobulin class mediates this hemolytic anemia?
- A IgG
- B IgM ✓
- C IgA
- D IgE
Correct answer: B. IgM
Explanation
Cold agglutinin disease is mediated by IgM antibodies that react optimally at 4°C, fixing complement (C3d) on red cells. DAT is positive for C3d but negative for IgG. Warm AIHA is IgG-mediated with DAT positive for IgG +/- C3d. IgA and IgE are not implicated in autoimmune hemolytic anemias.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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