H antigen is expressed on a mixture ofN- andO-linked glycans on major platelet GPs. Potential H epitopes on the 3 major platelet GPs: GPIIb, GPIIIa, and GP1b. On GPs IIb and IIIa, there is a mixture of high mannose, biattenary, and triattenary type 2 chain N-glycans.⁵⁵  It is estimated that 8% to 10% of biattenary and triattenary structures may terminate in an H-active epitope, which could be further modified to A and B antigens in group A and B individuals, respectively. Based on the number of GPIIb/IIIa molecules per platelet,⁵⁴  this could equal approximately 100 000 ABH epitopes/platelet. GPIb, a sialomucin, has limited N-glycans but is rich in O-linked carbohydrate, which comprises 60% of the molecular weight of the molecule.⁵⁴  Most of the O-linked carbohydrate exists as a disialo, branched core 2 oligosaccharide.^56-58  A minor component (4%) is a monosialo, fucosylated derivative with H-like activity⁵⁷  that could number nearly 75 000 H epitopes/platelet. Similar O-glycan structures may exist on GPIV.⁵⁹  Because of their branched structure, and the density of O-glycan on GPIb molecule, these H-like epitopes may be sterically inaccessible for further modification by the A/B glycosyltransferase. As a consequence, increases in H-fucosyltransferase activity could result in parallel increases in H and A antigen on O- and N-linked glycans, respectively.