PLTW Human Body Systems Practice Test 2025 – Complete Exam Prep

Blood type O is considered the universal donor because it lacks A and B antigens on the surface of its red blood cells. This means that when a person with type O blood donates to individuals with any other blood type (A, B, AB, or O), their blood will not trigger an immune response, as there are no foreign antigens to be recognized by the recipient's immune system. As a result, type O blood can be safely transfused to anyone, making it crucial in emergency situations where the blood type of the patient is unknown.

In contrast, blood types A, B, and AB contain specific antigens that could provoke an immune reaction if transfused to someone whose blood does not match those antigens. For example, type A blood has A antigens that would be recognized as foreign by type B or O recipients, and type B blood has B antigens that would be problematic for type A or O recipients. Therefore, type O's absence of these antigens is what solidifies its role as the universal donor.