A 34-year-old man recovering from atypical pneumonia develops fatigue and dusky discoloration of his fingertips and earlobes on cold exposure. Hb is 8.4 g/dL, reticulocytes 7%, LDH elevated, haptoglobin undetectable. The blood sample clumps spontaneously in the laboratory tube at room temperature but disperses on warming to 37 degrees C. The antibody responsible is best characterized as:

• A IgG directed against Rh antigens, active at 37 degrees C
• B IgA directed against band 3 protein, complement independent
• C IgG directed against P antigen, biphasic thermal amplitude
• D IgM directed against the I antigen, maximally active below 30 degrees C ✓

Explanation

Cold agglutinin disease following Mycoplasma pneumoniae infection is caused by IgM antibodies against the I/i antigen on red cells. These pentameric IgM molecules bind below 30 degrees C, fix complement, and cause both acrocyanosis and complement-mediated hemolysis; agglutination reverses on warming. Option A describes warm AIHA, option C describes the Donath-Landsteiner antibody of paroxysmal cold hemoglobinuria, and cold agglutinins are classically complement dependent, ruling out B.

Reference: Williams Hematology, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.