The reasoning: antigen on the cell, antibody in the plasma
ABO antigens are carbohydrates built on the H precursor (FUT1): A transferase adds N‑acetylgalactosamine; B adds galactose; O lacks a functional transferase and retains H. Naturally occurring anti‑A/anti‑B (mostly IgM, complement‑fixing) appear from ~3–6 months of age after exposure to cross‑reacting gut‑bacterial antigens. Bombay phenotype (Oₕ) lacks H on red cells and makes potent anti‑H, anti‑A and anti‑B — incompatible with all ordinary donors, including group O.