Reply to comment

Although the misconception that the A and B antigens are proteins is widespread, it is now well established that this is not true. "The gene that determines ABO ... encodes an enzyme that adds sugar molecules to lipids on the surface of red blood cells. ... The gene that encodes the enzyme, designated I, has three common alleles: I^B whose product adds a lactose; I^A whose product adds galactoseamine; and i which codes for a protein that does not add a sugar.... If a type A individual receives a transfusion of type B blood, the recipient's immune system recognizes that the type B blood cells possess a "foreign" antigen (galactose) and attacks that donated blood cells, causing them to clump, or agglutinate." (quoted from Raven et al., Biology, seventh edition, page 260).  Additional supporting information can be found in immunology textbooks.