A universal blood donor is someone whose blood type is O negative. Blood type is a designation of two sets of proteins (called antigens) on the outside of red blood cells. One antigen type is the A, B, O type and the other is the Rh type (either positive or negative).
O Negative blood cells are called “universal” meaning they can be transfused to almost any patient in need, and only 6.6% of the population has O Negative blood. In the event of an emergency, trauma patients and accident victims are given a fighting chance at life due to O Negative blood transfusion.
Type O blood lacks antigens on the outside of the red blood cells. This means that our immune system will not react to the blood because there is no antigen to react against. Similarly, Rh negative blood lacks the Rh antigens on the outside of the blood cells, so once again there are no proteins for our immune system to react against. This lack of antigens means that type O negative blood can be transfused in people of any blood type; hence, the term universal blood donor.