## Rh Blood Group System: Immunology and Clinical Significance **Key Point:** Rh-negative individuals do NOT naturally possess anti-D antibodies. Anti-D develops only after sensitization through transfusion of Rh-positive blood or during pregnancy/delivery with an Rh-positive fetus. ### Mechanism of Rh Sensitization 1. First exposure to Rh-positive RBCs → primary immune response (IgM, weak or absent) 2. Second exposure → secondary immune response (IgG antibodies develop over weeks to months) 3. IgG anti-D crosses the placenta → hemolytic disease of the newborn in subsequent pregnancies **High-Yield:** Unlike ABO (natural IgM antibodies), Rh incompatibility requires prior sensitization and causes delayed hemolytic transfusion reactions via IgG antibodies. ### Why Each Statement Is Correct or Incorrect | Statement | Correct? | Reason | |-----------|----------|--------| | Rh+ have natural anti-D | ✗ | Anti-D is NOT naturally occurring; it requires sensitization | | Rh− develop anti-D after exposure | ✓ | Correct — primary and secondary immune responses occur after transfusion or pregnancy | | D antigen causes 99% of Rh incompatibility | ✓ | True, but this is not the focus of the question | | Rh incompatibility = immediate hemolysis | ✗ | Rh incompatibility causes delayed hemolysis (IgG-mediated); ABO causes immediate hemolysis | **Clinical Pearl:** Rh-negative women of childbearing age should receive anti-D immunoglobulin (500 IU/mL or 100 μg/mL) within 72 hours of delivery, miscarriage, or abortion to prevent sensitization and hemolytic disease in future pregnancies. **Mnemonic:** **ANTI-D DELAY** = **A**ntibodies (IgG, not natural), **N**eed sensitization, **T**ake weeks to develop, **I**ntravascular hemolysis (delayed), **D**elivery risk in next pregnancy.
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