Rh-antigen is present on the surface of erythrocytes in about 80-85 % of the human beings.
The individuals, who possess this antigen are called + (Rh positive) and those, who do not have it are called “ (Rh negative) “ person, when exposed to + blood, develops anti Rh-antibodies.
A pregnant women who is - if bears an + foetus, will develop anti Rh-antibodies during the
first delivery, when the foetal blood comes in contact with her blood. These antibodies linger in the blood for sufficiently longer periods. If she carries a second foetus, that is +, the anti-Rh-antibodies in her blood enter the foetal circulation and cause damage to the foetal RBCs.
This, could become fatal. This condition is called erythroblastosis foetalis.