A 5-year-old boy develops dark urine and pallor one week after recovery from an upper respiratory infection. Smear shows spherocytes and polychromasia. Direct Coombs test is positive for complement only. Incubation of his serum with red cells shows hemolysis after cooling then rewarming. The responsible antibody has specificity against which antigen?

• A Kell antigen
• B I/i antigen
• C Rh antigen
• D P antigen ✓

Explanation

Paroxysmal cold hemoglobinuria is caused by the Donath-Landsteiner antibody, an IgG that binds red cells in the cold, fixes complement, and lyses them on rewarming, giving the biphasic hemolysis test described. It is directed against the P antigen and classically follows viral infections in children. Cold agglutinin disease antibodies are IgM directed against I/i antigens, and they cause acrocyanosis rather than this biphasic pattern.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.