Anti-I may be present in which condition?

• A. Infectious mononucleosis
• B. Systemic lupus erythematosus
• C. Pernicious anemia
• D. Iron deficiency

Answer: (A)

Anti-I is a cold-reacting autoantibody directed against the I antigen on adult red blood cells. It is classically seen in infectious mononucleosis as a result of polyclonal B‑cell activation during the infection, leading to production of cold agglutinins such as anti-I. This association is the reason it may be detected in mono, sometimes producing a positive cold agglutinin test or mild hemolysis at low temperatures. The other conditions listed do not have a strong link to anti-I: systemic lupus erythematosus involves a variety of autoantibodies but anti-I is not particularly characteristic; pernicious anemia involves anti-intrinsic factor or anti-parietal cell antibodies; iron deficiency is not associated with anti-I antibodies.