A schematic comparison between ABO and GPIb glycans. Glycans on GPIbα are most likely to be heavily modified by terminal sialic acids, whereas platelet ABO is less likely to be sialylated. There are 2 possible locations of ABO epitopes on platelets: the mucin-rich region of GPIbα and membrane glycolipids. Neither is in the VWF-binding region, suggesting that ABO indirectly regulates GPIb-VWF interaction. The 2 potential locations of ABO on platelets could also lead to different mechanisms of regulating GPIb-VWF interaction in flowing blood.