Which of the following blood types is considered to be a universal donor?

The blood type that is considered to be a universal donor is O-. This designation comes from the characteristics of the type O blood group, particularly the absence of A and B antigens on the surface of red blood cells. Because O- blood lacks these antigens, it does not trigger an immune response in individuals who may have different blood types when they receive a transfusion.

In practical terms, this means that O- blood can be administered to patients of any other blood type without the risk of hemolytic reactions, where the recipient's immune system would attack the transfused red blood cells. Therefore, O- donors are especially vital in emergency situations when immediate blood transfusions are necessary and the blood type of the recipient might not be determined quickly.

Moreover, while other blood types like O+ can also be donated to many individuals, they still carry the Rh factor, which can complicate transfusions for patients who are Rh-negative. Hence, O- remains the safest choice for universal donations.