Inhibition of the binding of anti-A IgG of an individual of blood group A and of an individual of blood group B, to A antigen by soluble A antigen. Anti-A IgG of an individual of blood group A (left panel) and anti-A IgG of an individual of blood group B (right panel) were coincubated with varying concentrations of inhibitors soluble A antigen, B antigen, and H substance. Antibody binding was determined using an ELISA.