Answer:
A special case of Rh incompatibility (mismatching) has been observed between the Rh-ve blood of a pregnant mother with Rh+ve blood of the foetus. The Rh-ve mother is not usually exposed to the Rh antigen of the foetus during the first pregnancy as the foetal and maternal blood are normally kept well seperated by the placental barriers. However, at the time of birth of first child, there is a possibility of exposure of the maternal blood to small amount of the Rh+ve blood from the foetus. In such cases, the mother starts preparing antibodies against Rh antigen in her blood.
These antibodies could cross the placental barriers in subsequent pregnancies and cause hemolysis of the Rh+ve red blood cells of the foetus. This could be fatal to the foetus or could cause severe anaemia and jaundice to the baby (HDN - Haemolytic disease of newly born). This condition is called erythroblastosis foetalis.
Erythroblastosis foetalis can be prevented by injecting the Rh-ve mother with an antibody preparation (anti-Rh antibodies) against the Rh factor immediately after the birth of each Rh+ve baby.