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In the ABO blood typing system, which blood type is considered the "universal donor"?

  1. Type A

  2. Type B

  3. Type AB

  4. Type O

The correct answer is: Type O

Type O is considered the "universal donor" in the ABO blood typing system because it lacks A and B antigens on the surface of its red blood cells. This characteristic allows type O blood to be transfused into individuals with any other blood type without triggering an immune response. When a person with type A, B, or AB blood receives type O blood, their immune system does not react against the absence of A and B antigens, thereby minimizing the risk of hemolytic transfusion reactions. This is particularly crucial in emergency situations where matching a patient's specific blood type may take time. In contrast, other blood types contain specific antigens that can provoke an immune response if transfused improperly. Therefore, type O is favored in transfusion medicine as it is the safest option for the recipient of any blood type.