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 when a Rh- mother gets exposed to her babies Rh+ blood. This usually happens at delivery (so Baby 1 is usually healthy). Baby 2 (and subsequent) is now at risk because the mother has made anti-Rh antibodies. These destroy the red cells of a subsequent Rh+ foetus.
An Rh negative mom has no Rh antigens in her body though these antigens are found in the blood of an Rh positive baby. Antibodies of mom’s blood fail to identify the Rh antigens of her baby. When mom’s antibodies go across thru the placenta, they identify antigens of fetal blood as foreign and begin their defensive action. This may hurt the baby. The situation is not severe in the case of the first baby. When another Rh positive baby is created, the antibodies contradictory to Rh antigen are previously in existence in the body of the mom and their effect is obvious. The outcome could be demise of the fetus.