If a person is blood type O and needs a transfusion, what blood type could act as a donor?

A person with blood type O has no A or B antigens on the surface of their red blood cells. This means that they can only receive blood from donors who also have type O blood, which is why type O is commonly referred to as the universal donor for plasma, but not for red blood cells. For red blood cells, since O type lacks both antigens, it is the only compatible type for a person with O blood.

Transfusing blood from any other type, such as A, B, or AB, would introduce foreign antigens that the immune system of a type O individual would recognize as invaders, potentially leading to a serious immune reaction. Thus, in a scenario where a person with type O blood requires a transfusion, only type O blood is safe and appropriate for them to receive.