Rh-isoimmunization Notes

Two main blood group systems: ABO and Rhesus (Rh) factor. The Rh system is more complex, but the D antigen is the most significant for transfusion purposes.
Epidemiology of Rh (-) women:
- Caucasian: 15%
- African-Americans (Blacks): 5-10%
- Asian: < 5%
Rhesus (+) men:
- 55% are heterozygous (Dd): 50% chance of Rh+ fetus. This means that the father has one Rh+ allele (D) and one Rh- allele (d), so there is a 50% chance of passing on the Rh+ allele.
- 45% are homozygous (DD): 100% chance of Rh+ fetus. This means that the father has two Rh+ alleles (DD), so the fetus will always inherit the Rh+ allele.

Mother is Rh (-) and fetus is Rh (+). This is the necessary condition for Rh-isoimmunization to occur.
Isoimmunization: fetal blood exposure to mother's blood (fetomaternal hemorrhage). This can happen during pregnancy, delivery, or certain medical procedures.
Mother develops IgM (cannot cross the placenta) initially. IgM antibodies are produced first in response to the Rh+ antigens but are too large to cross the placenta and affect the fetus in the first pregnancy.
Subsequent pregnancy (Rh+ fetus):
- Mother develops IgG (can cross the placenta). IgG antibodies are smaller and can cross the placenta, posing a threat to subsequent Rh+ fetuses.
- Extravascular hemolytic anemia occurs in the fetus. IgG antibodies bind to the Rh+ antigens on the fetal red blood cells, leading to their destruction in the spleen.
Risk of sensitization depends on:
- Volume of transplacental hemorrhage. The larger the volume of fetal blood that enters the maternal circulation, the higher the risk of sensitization.
- Extent of the maternal immune response. Some women are more prone to developing a strong immune response to Rh+ antigens.
- Presence of ABO incompatibility (protective). If the mother and fetus have ABO incompatibility, the mother's immune system will target the fetal red blood cells for destruction before Rh sensitization can occur.