## Blood Group Antigens and Immunogenicity **Key Point:** The D antigen (RhD) is the most immunogenic blood group antigen after the ABO system. It is a transmembrane glycoprotein present on the red cell surface. ### Why D Antigen is Most Immunogenic 1. **Structure**: D antigen is a transmembrane protein with multiple epitopes, making it highly antigenic. 2. **Antibody Response**: Even a single transfusion of D-positive blood to a D-negative recipient can sensitize the immune system. 3. **Clinical Significance**: Anti-D antibodies develop in ~80% of D-negative individuals after exposure to D-positive RBCs, leading to hemolytic transfusion reactions and hemolytic disease of the newborn (HDN). ### Comparison of Blood Group Antigens | Antigen | System | Immunogenicity | Naturally Occurring Antibodies | Clinical Significance | |---------|--------|---|---|---| | D (Rh) | Rh | Very high | No (requires sensitization) | Hemolytic transfusion reactions, HDN | | ABO | ABO | Highest | Yes (naturally present) | Acute hemolytic transfusion reactions | | Kidd | Kidd | Moderate | Rare | Delayed hemolytic transfusion reactions | | Duffy | Duffy | Low | Rare | Minimal clinical significance | | MNS | MNS | Low | Rare | Minimal clinical significance | **High-Yield:** Among non-ABO antigens, the D antigen is the most important for transfusion medicine. Rh-negative individuals must receive Rh-negative blood products to prevent alloimmunization. **Clinical Pearl:** A D-negative woman of childbearing age transfused with D-positive blood can develop anti-D antibodies, which may cause hemolytic disease in future pregnancies if the fetus is D-positive. [cite:Harrison 21e Ch 180]
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