## Rh Blood Group System: Key Distinctions ### Correct Statements (Options 0, 1, 2) **Option 0 — Anti-D prophylaxis after delivery** - This is TRUE. An Rh-negative woman carrying an Rh-positive fetus risks fetomaternal hemorrhage (FMH) during delivery. - Anti-D immunoglobulin (500 IU per mL fetal RBCs or standard 100–120 µg dose) prevents primary sensitization. - Without prophylaxis, subsequent Rh-positive pregnancies face risk of hemolytic disease of the newborn (HDN). **Option 1 — Rh-positive blood in Rh-negative recipients** - This is TRUE. In life-threatening emergencies, Rh-positive blood may be given to Rh-negative recipients. - First transfusion rarely causes acute hemolysis (no pre-formed anti-D IgM). - However, the recipient becomes sensitized and will produce anti-D IgG; subsequent Rh-positive transfusions risk delayed hemolytic transfusion reaction. **Option 2 — Rh antigen is a protein** - This is TRUE. The Rh antigen is a transmembrane protein (RhD protein), NOT a carbohydrate like ABO antigens. - This structural difference explains why Rh antibodies develop only after exposure (sensitization), whereas ABO antibodies are naturally occurring. ### Incorrect Statement (Option 3) — The Correct Answer **Option 3 — Naturally occurring anti-D antibodies in Rh-negative plasma** - This is FALSE. Unlike anti-A and anti-B antibodies in Type O (and other ABO-incompatible) individuals, **anti-D antibodies are NOT naturally occurring**. - Anti-D antibodies develop only after sensitization (transfusion of Rh-positive blood, pregnancy with Rh-positive fetus, or other fetal-maternal hemorrhage). - An Rh-negative individual who has never been transfused and has no obstetric history will have NO anti-D antibodies. - This is the critical distinction between ABO and Rh systems: ABO antibodies are innate; Rh antibodies are acquired. ### High-Yield Comparison Table | Feature | ABO System | Rh System | | --- | --- | --- | | **Antigen Structure** | Carbohydrate (glycoprotein) | Protein (transmembrane) | | **Naturally Occurring Antibodies** | Yes (IgM + IgG from birth/infancy) | No (only after sensitization) | | **Sensitization Required** | No | Yes | | **First Incompatible Transfusion** | Acute hemolysis (IgM) | Usually safe (no pre-formed IgG) | | **Repeat Incompatible Transfusion** | Acute hemolysis | Delayed hemolysis (IgG) | | **Hemolytic Disease of Newborn** | Possible (especially Type O mother → A/B baby) | More severe (especially Rh-negative mother → Rh-positive baby) | **Mnemonic: CARB vs PROTEIN** - **ABO = CARBohydrate** → naturally occurring antibodies (innate immunity) - **Rh = PROTEIN** → no natural antibodies (acquired immunity only) **Key Point:** The absence of naturally occurring anti-D antibodies is why Rh-negative individuals can receive their first Rh-positive transfusion in emergencies without immediate hemolysis, but why anti-D prophylaxis is critical to prevent sensitization. **Clinical Pearl:** An Rh-negative woman who has never been pregnant or transfused is "unsensitized." Her first Rh-positive pregnancy or transfusion carries a 10–15% risk of sensitization. Anti-D immunoglobulin within 72 hours of delivery reduces this risk to <1%.
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