Sialic acids occupy the terminal position of glycan chains and have the potential to influence the antigenicity of glycoproteins. The polymorphisms of Human Platelet Alloantigens (HPA)-3 and HPA-9, locating near the C-terminus of the extracellular domain of platelet membrane glycoprotein (GP)IIb, are adjacent to sialyl-Core 1 O-glycans emanating from serines 845 and 847. Whether the nearby O-glycans affect the antigenicity of HPA-9b, and/or influence the binding of anti-HPA-9b alloantibodies …