Can an Rh Positive Mother Get an Allergic Reaction from an Rh Negative Baby?
No, an Rh positive mother cannot develop an immune response to an Rh negative baby because the mother already possesses the Rh antigen, and the immune system does not react against antigens that are absent in the fetus.
Understanding the Immunologic Mechanism
The concern about Rh incompatibility only flows in one direction—from Rh negative mothers carrying Rh positive babies. Here's why:
Rh alloimmunization occurs when an Rh negative mother is exposed to Rh positive fetal red blood cells, which display the RhD antigen that the mother's immune system recognizes as foreign 1, 2.
An Rh positive mother already has the RhD antigen on her own red blood cells, so her immune system is tolerant to this antigen and will not mount an immune response against it 3.
The fetus can only "give" antigens to the mother through fetomaternal hemorrhage—it cannot take them away. An Rh negative baby simply lacks the RhD antigen, meaning there is no foreign antigen for the Rh positive mother's immune system to react against 2.
The Actual Clinical Scenario of Concern
The question appears to have the incompatibility reversed. The clinically significant scenario is:
Rh negative mothers carrying Rh positive babies are at risk, with fetal RBCs displaying the D-antigen as early as 6 weeks of gestation 1, 4.
Without prophylaxis, approximately 17% of Rh negative women become alloimmunized after delivering an ABO-compatible, Rh positive infant 2.
Fetomaternal hemorrhage occurs in approximately 50% of all deliveries, allowing fetal red cells to enter maternal circulation, and as little as 0.1 mL of Rh positive fetal cells can cause alloimmunization 2.
Why the First Baby Is Usually Unaffected
Approximately 90% of fetomaternal hemorrhage and alloimmunization occur during delivery, meaning the first baby is typically born before significant maternal antibody production occurs 4.
The first Rh positive baby born to an Rh negative mother rarely develops hemolytic disease because maternal sensitization hasn't yet occurred 4, 5.
Subsequent pregnancies carry the real risk, as the anamnestic immune response produces higher antibody levels that can cross the placenta and cause hemolytic disease of the fetus and newborn 6.
Prevention Strategy (For the Correct Scenario)
If you are managing an Rh negative mother:
Administer RhIg 300 μg at 28 weeks gestation and within 72 hours postpartum if the baby is Rh positive, reducing alloimmunization from 12-13% to 0.1-0.2% 1, 3.
For first trimester events (abortion, bleeding), give 50 μg RhIg within 72 hours before 12 weeks, or 300 μg after 12 weeks 1.
RhIg works by suppressing the maternal immune response to Rh positive fetal red blood cells, though the complete mechanism is not fully understood 3.