Which blood type is known as the universal donor?

The designation of Type O as the universal donor is based on its compatibility with other blood types during transfusions. Type O blood lacks A and B antigens on the surface of its red blood cells. This absence means that Type O blood can be transfused to individuals with any other blood type (A, B, AB, or O) without the risk of triggering an immune response, as the recipient's body will not recognize the Type O cells as foreign.

In contrast, blood types A and B produce respective antibodies against the antigens present in other types, and Type AB has both A and B antigens and thus can only receive A, B, or O blood. This specific characteristic of Type O is what grants it the title of universal donor, making it particularly valuable in emergencies when blood type matching may not be possible.