Anti-i may be present in the serum of patients with which clinical condition?

• A. infectious mononucleosis
• B. systemic lupus erythematosus
• C. sickle cell anemia
• D. thrombotic thrombocytopenic purpura

Answer: (A)

Anti-i is a cold-reactive autoantibody that targets i antigen on red cells. It is most classically seen in infectious mononucleosis caused by EBV, where polyclonal B-cell activation leads to production of this autoantibody. Because it reacts best at low temperatures, it can cause spontaneous agglutination of i-positive red cells in serologic testing done at cooler temps, which is a distinctive clue for this infection. The other conditions listed are not characteristically linked to anti-i antibodies—sickle cell anemia and thrombotic thrombocytopenic purpura don’t typically involve anti-i, and while SLE can have various autoantibodies and cold agglutinins, anti-i is not a hallmark feature there.