A case of Rh incompatibility is seen in pregnancy if Rh negative mother is pregnant with Rh positive child. During the first pregnancy of Rh negative mother with Rh positive child, the mother's body develops B cell antibodies because mother's blood gets mixed up with foetal blood during child birth. The birth of the first child is normal. Complications develop if the second child is also Rh positive. The Rh antibodies in mother's blood react with Rh factor in foetal blood. This results in destruction of RBCs in foetus and may result in death of the child. If the child is born he/she is very weak and suffers from hepatitis. This condition is called erythroblastosis foetalis. To prevent this, the mother is given a vaccine within 24 hours of birth of the first child. These are called Rhogam antibodies.