Erythroblastosis foetalis can be prevented by administering anti-Rh antibodies to the Rh-ve mother immediately after the delivery of the first Rh +ve child because

Erythroblastosis foetalis can be prevented by administering anti-Rh antibodies to the mother immediately after the delivery of the first child. These antibodies bind to and inactivate the fetal Rh antigens before the mother’s immune system can respond to the foreign antigens by producing her own anti-Rh antibodies. This is an example of passive immunisation. Anti-Rh antibodies (IgG) are small and can cross the placenta.