Real time analysis of the binding of anti-A antibodies to immobilized A antigen. Top panels: apparent equilibrium constants (K_A) for the binding of purified IgG from serum of individuals A1, B, AB, and O (left panel) and for the binding of affinity-purified anti-A IgG from individuals A1 and B (right panel) to A antigen. Lower panels: mole to mole ratios of bound antibody to antigen for the binding to A antigen of IgG of individuals A1, B, AB, and O (left panel) and for the binding to A antigen of affinity-purified anti-A IgG from individuals A1 and B (right panel).