Rh disease is caused by the irruption in the fetal circulation of anti-Rh(D) antibodies. This only happens when there has been a prior exposure of the mother to Rh+ RBC. To prevent Rh sensitization during pregnancy, anti-Rh(D) antibodies are typically administered during pregnancy.
The way anti-Rh(D) work in preventing Rh sensitization has been studied extensively: in short, it seems that they cover fetal RBC that have crossed the placenta, thereby preventing the mother's immune system from recognizing the Rh antigens. However, I have not been able to find a satisfying answer to the following question: given that anti-Rh(D) use in the prevention of Rh disease are the same antibodies that cause Rh disease, and given in particular that they do cross the placenta, how does it come that they have no harmful effect on the fetus?


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