Which statement concerning anti-I is incorrect?

• A. it reacts best with cord red blood cells
• B. it is a complement binding antibody
• C. it is detected at room temperature
• D. it is sometimes clinically insignificant

Answer: (A)

Anti-I is a cold agglutinin directed against the I antigen on red cells. The I antigen is abundant on mature adult red cells, while cord blood cells predominantly express the related i antigen and have little I antigen. Because of this, anti-I does not bind well to cord red blood cells, so the statement that it reacts best with cord cells is incorrect. In typical circumstances, anti-I is a cold-reactive IgM antibody that can fix complement and is often detected at room temperature or colder. Clinically it is frequently insignificant, though in some situations it can contribute to hemolysis if cold exposure occurs.