Rh grouping

Besides the ABO blood groups, human blood is classified by another important antigen on the surface of the red blood cells — the Rh antigen, so called because an antigen very like it was first observed in the rhesus monkey. About 80 per cent of people carry this antigen and are said to be Rh positive (Rh+), while those who lack it are Rh negative (Rh−).

Why Rh matching matters in transfusion

Rh grouping must be matched before a transfusion, just as ABO groups are. If the blood of an Rh− person is exposed to Rh+ blood, that person's body treats the Rh antigen as foreign and begins to make anti-Rh antibodies against it. A later exposure to Rh+ blood would then trigger a harmful reaction as these antibodies attack the transfused red cells.

Rh incompatibility between mother and foetus (erythroblastosis foetalis)

A special and serious case arises during pregnancy. If an Rh− mother is carrying an Rh+ foetus (having inherited the antigen from an Rh+ father), the two bloodstreams are normally kept apart by the placenta, so the first pregnancy is usually safe. However, at the time of delivery some of the foetal Rh+ blood can leak across into the mother's circulation, and she then starts to form anti-Rh antibodies. …