Blood groups (ABO and Rh)

ABO antigens. All ABO antigens are built on a carbohydrate called the H antigen. The A allele codes for an enzyme that adds N-acetylgalactosamine to it. The B allele codes for an enzyme that adds galactose. The O allele codes for no working enzyme, so group O cells carry only the H antigen. A and B are codominant, and O is recessive.

ABO antibodies. In the first months of life, people make IgM antibodies against whichever A or B antigens they lack, probably triggered by similar molecules on gut bacteria. Because these antibodies are already present, an ABO-mismatched transfusion causes immediate destruction of red cells inside the blood vessels. Group O red cells can be given to anyone (universal red-cell donor). Group AB people can receive red cells of any group (universal recipient). For plasma the rules are reversed: AB plasma, which contains no antibodies, can be given to anyone.

Rh system. The Rh system contains more than 50 antigens, but D is the most likely to provoke an immune response. About 85% of people of European descent are Rh(D)-positive, and the proportion is higher in African and East Asian populations. Anti-D is not present naturally. It forms only after a D-negative person is exposed to D-positive cells, through transfusion or pregnancy. Anti-D is IgG, so it can cross the placenta.

Other systems. Other clinically important blood group systems include Kell, Duffy (its antigen is the receptor that Plasmodium vivax uses to enter red cells), Kidd and MNS. People with the rare Bombay (hh) phenotype lack the H antigen altogether.