ABO Antigens: Found on RBCs AND other tissues (endothelium, platelets, fetal tissues).
Result: Maternal Anti-A/B antibodies crossing the placenta bind to these other tissues, effectively “soaking up” or neutralizing the antibodies before they can significantly destroy RBCs.
Rh Antigens: Found ONLY on RBCs.
Result: Every maternal Anti-D antibody that crosses the placenta targets red blood cells specifically, leading to focused and massive hemolysis.
2. Antigen Maturity
ABO Antigens:Weakly expressed on fetal RBCs compared to adult RBCs.
Result: Less binding sites for antibodies → less hemolysis.
Rh Antigens:Fully expressed at birth.
Result: High density of binding sites allows for rapid destruction (extravascular hemolysis in the spleen).
ABO incompatibility
Highest risk: mother with blood group O; newborn with blood group A or B
Maternal antibodies (anti-A and/or anti-B) against nonself antigens of the ABO system are present even if sensitization has not occurred, so fetal hemolysis may occur during the first pregnancy.
Combination of predominantly IgM antibodies and late expression of fetal ABO antigens reduces the chances of significant disease.
Clinical features
Diagnostics
Treatment
Prevention (Rh only): Administer Rho(D) immune globulin (RhoGAM) to Rh-negative mothers at 28 weeks gestation and within 72 hours of delivery of an Rh-positive infant.
Administered anti-D IgG binds to fetal Rh(+) RBCs in maternal circulation. Opsonized fetal RBCs are cleared by maternal splenic macrophages before maternal B-cells can recognize the D-antigen.
Fetal Tx: Intrauterine blood transfusion if severe anemia is detected via Doppler of the middle cerebral artery (↑ peak systolic velocity).
Neonatal Tx:
Phototherapy: Converts unconjugated bilirubin to water-soluble isomers.
Exchange Transfusion: Used if bilirubin levels are dangerously high despite phototherapy.