Blood groups

Human blood looks alike but differs in certain respects; the two groupings used worldwide are ABO and Rh.

ABO grouping depends on whether two surface antigens, A and B, are present on the RBCs. Antigens are chemicals that can trigger an immune response. Plasma carries natural antibodies, proteins made in response to antigens.

Group A: antigen A on RBCs, anti-B in plasma, can receive from A and O. Group B: antigen B, anti-A, can receive from B and O.

Group AB: both antigens, no antibodies, can receive from AB, A, B and O. Group O: no antigens, both anti-A and anti-B, can receive only from O.

Donor and recipient blood must be matched before a transfusion; a mismatch causes clumping, which destroys RBCs.

Group O can be given to people of any group, so O individuals are universal donors. Group AB can receive from every group, so AB individuals are universal recipients.

Rh grouping: an antigen similar to one in Rhesus monkeys, the Rh antigen, is found on the RBCs of nearly 80 per cent of humans (Rh positive); people without it are Rh negative.

An Rh negative person exposed to Rh positive blood makes antibodies against the Rh antigen, so Rh group must also be matched before a transfusion.

Rh incompatibility in pregnancy: in the first pregnancy the placenta keeps the blood of an Rh negative mother apart from that of an Rh positive foetus, but at delivery a little foetal blood may reach the mother, who then starts making anti-Rh antibodies.

In later pregnancies these antibodies can leak into the blood of an Rh positive foetus and destroy its RBCs, which may be fatal or cause severe anaemia and jaundice in the baby: erythroblastosis foetalis. It can be prevented by giving the mother anti-Rh antibodies immediately after the first delivery.