Which statement about anti-I is incorrect?

• A. It reacts best with cord red blood cells
• B. It is a complement binding antibody
• C. It may be detected at room temperature
• D. It is a cold antibody

Answer: (A)

Anti-I is a cold agglutinin, typically an IgM antibody, that targets the I antigen on red blood cells. The I antigen is expressed more on adult red cells, while cord blood cells predominantly carry the related i antigen and have little I antigen. Because of this, anti-I does not bind well to cord cells and actually shows the strongest reactivity with adult RBCs. It can fix complement and is most active at lower temperatures, so it may be detected at room temperature in tests. The statement about reacting best with cord red blood cells is therefore incorrect. The other aspects—being a cold antibody and capable of complement binding, with detection at room temperature—are correct.