Reactivity with A antigen of affinity-purified anti-A antibodies isolated from the serum of donors of group A1 and of group B. Anti-A antibodies were affinity-purified from pooled serum of six individuals belonging to each blood group category by chromatography on a macroporous glass matrix covalently coated with synthetic A trisaccharide. Anti-A IgG was then separated from anti-A IgM by chromatography on protein G Sepharose. Left panel: Binding of anti-A IgG of A1 (□) and B (⧫) individuals to A antigen, as assessed by ELISA. Right panel: Binding of anti-A IgM of A1 (□) and B (⧫) individuals to A antigen, as assessed by ELISA.