During erythroblastosis fetalis, a rh− mother's anti-rh antibodies that have crossed the placenta will cause agglutination of the fetus's rh+ rbcs. however, the reverse problem never happens when a rh+ mother is pregnant with a rh− fetus; that is, antibodies produced by the fetus cannot cause agglutination of the mother's rh+ rbcs. this is true because

The correct answer is - fetus does not form antibodies due to an immature immune system.

Explanation:

Erythroblastosis details are a kind of anemia which is caused by transplacental transmission of antibodies of mother to the blood (red blood cells) to which results in incompatibility in their blood group mostly due to Rh factor or antigen.

However, in reverse, it is not the case because of the immature immune system which does not produce antibodies that can cause incompatibility between the antigen of mother blood.

Thus, the correct answer is - the fetus is not able to produce antibodies due to the immature immune system.