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 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 agglutinins are physically too large to pass across the placenta fetal antibodies are immature and non-functional antibodies that can cause this agglutination are not produced by a fetus the placenta is a barrier that prevents the passage of all antigens