Model for clustered sialic acid patches on RBCs of different blood groups. Sialylated glycans and ABH blood group antigens are organized in clusters on RBC membranes. These clusters are stabilized by carbohydrate-carbohydrate interactions with the type A and B terminal saccharides. (Top) On blood type A RBCs, the A antigens are localized at the periphery of the cluster, whereas Sias form a continuous cluster that yields a local increase in the Sia concentration. (Middle) On blood type B RBCs, the B antigen is localized in the center of the Sia cluster in approximately 60% of the clusters. (Bottom) On blood type O RBCs, the Sias clusters are relatively small and may not include the H antigen.