The ABO system is a group of carbohydrate antigens in which the individual alleles are defined by the terminal saccharide moiety. Specifically, addition of N-acetylgalactosamine or galactose to the subterminal galactose yields red blood cells (RBCs) of group A or group B, respectively. Individuals who express neither of these sugars on the subterminal galactose are group O, and individuals who express both sugars are group AB. The subterminal galactose, in association with a constitutively expressed fucose moiety, defines the so-called H antigen. As such, some authors refer to the ABO antigen system as the ABH system. For reasons that . . . [Full Text of this Article]