Blood Groups

Two blood-typing systems are used almost everywhere in the world: ABO and Rh. In the ABO system, every person's red cells carry one of four antigen (agglutinogen) patterns on their surface, giving blood groups A, B, AB or O, while the plasma simultaneously carries natural antibodies (agglutinins) against whichever antigen the person's own cells lack - so a person with no A antigen makes anti-A antibodies, and so on. Group A cells carry the A antigen and the plasma carries anti-B antibodies; group B cells carry the B antigen and the plasma carries anti-A antibodies; group AB cells carry both antigens and the plasma carries neither antibody; and group O cells carry no antigens at all, so the plasma carries both anti-A and anti-B antibodies (see Table 7.1). The A, B and O alleles are the major genes of the ABO system, and every agglutinogen shares a common sugar backbone (sucrose, D-galactose, N-acetyl glucosamine, plus terminal amino acids) whose final attachment - and therefore whether the antigen reads as A-type or B-type - is decided by the specific A or B gene product acting through the enzyme glycosyl transferase. The second system, Rh typing, is based on a single surface protein (the D antigen) present on the RBCs of about 80% of people; because this protein resembles one first found in Rhesus monkeys, carriers are called Rh1 (Rh positive) and non-carriers Rh2 (Rh negative). Rh compatibility must always be checked before a transfusion, and it matters just as much in pregnancy: if an Rh2 (Rh negative) mother carries an Rh1 (Rh positive) foetus, the two blood supplies stay separated by the placenta during a first pregnancy, but a small amount of foetal blood usually crosses over at birth, sensitising the mother's immune system to make anti-D antibodies. In a later pregnancy with …