Red blood cells of which blood group react most strongly with anti-H?

• A. A
• B. B
• C. AB
• D. O

Answer: (D)

H antigen density on red cells drives anti-H reactivity. The H antigen is the unmodified precursor to A and B antigens. In group O, there is no A or B modification, so the H antigen remains abundant on the cell surface, giving many targets for anti-H and yielding the strongest reaction. In groups that have A or B antigens, the H precursor is partly converted to A or B, reducing the amount of H antigen available, so the reaction with anti-H is weaker. AB has both A and B modifying the H antigen, further decreasing H units, and thus the reaction is not as strong as with group O. (Note: the Bombay phenotype lacks H antigen, so their cells do not react with anti-H, highlighting that the presence of H antigen, not just blood type label, matters for this reaction.)