A protein or glycoprotein found on the surface of a red blood cell (but often NOT ONLY on a red cell) and defined by recognition by a specific alloantibody. In other words, imagine you are a protein on a red cell. You do NOT get to claim the title of “blood group antigen” unless we have found an antibody in a person’s blood that reacts against you. There are TONS of blood group antigens; in fact, the ISBT recognizes over 325 of them at this time (so there are WAY more antigens than just ABO and Rh!). Most of the blood group antigens are organized into specific systems containing other antigens with a similar appearance or inheritance.

The above is not completely true, as some blood group antigens, most notably those in the Lewis System, float around free in the plasma most of the time. Still, Lewis antigens really only cause problems when they stick to the surface of a red cell, a process called adsorption, so the definition is reasonable.