A cold autoagglutinin will usually have specificity for which antigen?

• A. I antigen
• B. Ii antigen
• C. P antigen
• D. M antigen

Answer: (A)

Cold autoantibodies are typically IgM that react best at cold temperatures, and their classic target on red cells is the I antigen. This specificity explains the characteristic agglutination of red cells when tested at low temperatures and ties to conditions like Mycoplasma pneumoniae infection, which often stimulates anti-I production. The other antigens listed are not the usual targets of the common cold autoantibody: Ii/i represent another part of the same system but are not the typical target of these cold agglutinins, P antigen is involved in a different paroxysmal cold hemolysis scenario, and M antigen is a separate blood group antigen.