## Universal Donor and Recipient Concept **Key Point:** Blood group O is the universal donor because O red blood cells lack both A and B antigens, preventing hemolytic transfusion reactions in recipients of any ABO blood group. ### Why O is Universal Donor **High-Yield:** O RBCs have no A or B antigens → cannot trigger anti-A or anti-B antibodies in any recipient's plasma → safe for all ABO groups. ### ABO Compatibility Matrix | Donor Blood Group | Can Donate to | Reason | | --- | --- | --- | | O | O, A, B, AB | No A or B antigens present | | A | A, AB | A antigen present; lacks B | | B | B, AB | B antigen present; lacks A | | AB | AB only | Both A and B antigens present | **Clinical Pearl:** In emergency situations (massive transfusion, unknown blood type), O-negative blood is given because it lacks A, B antigens AND the Rh(D) antigen, making it safe for both ABO and Rh-incompatible recipients. ### Mechanism of Hemolytic Reaction If incompatible blood is transfused: 1. Recipient's naturally occurring antibodies (anti-A, anti-B) bind to donor RBC antigens 2. Complement activation and antibody-dependent cellular cytotoxicity occur 3. RBC destruction (hemolysis) results in acute hemolytic transfusion reaction **Mnemonic:** **DONOR O** = **O**nly one without antigens = can give to everyone.
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