Rh negative and Rh positive persons
People can be divided into two groups, differing in the presence of certain forms of a protein on the surface of the red blood cells. Rh positive persons (about 80% of the European population) have the relevant molecule on their red blood cells, while this molecule is missing in Rh negative persons (in actual fact, it is usually there, but is altered – however, that is not important here). If the blood of an Rh positive person is transferred to the body of an Rh negative person, the appropriate antibody molecules are formed and destroy the blood cells derived from the Rh positive person. Transfer of blood from an Rh positive person can occur during organ transplants or naturally in Rh negative women who expect an Rh positive child (with an Rh positive father). In the past, the presence of these antibodies seriously affected the lives and health of subsequent children of the same woman.