A protein named as rhesus antigen, is present on the surface of red blood corpuscles. Persons having this rhesus antigen (Rh factor) are called Rh positive (Rh+). Others who do not have this factor are known as Rh negative (Rh−). Both Rh+ and Rh− individuals are phenotypically normal. The problem arises during blood transfusion and pregnancy. The first blood transfusion of Rh+ blood to the person with Rh− blood causes no harm because the Rh− person develops anti Rh factors or antibodies in his\/her blood. In second blood transfusion of Rh+ blood to the Rh− person, the latter's anti Rh factors attack and destroy the red blood corpuscles of the donor. If father's blood is Rh+, mother's blood is Rh− and foetus' blood is Rh+. Then in second pregnancy (with Rh+ foetus), the anti Rh factors of the mother's blood destroy the fetal red blood corpuscles.