What is meant by a universal blood donor?

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). 

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.
Individuals with the blood group O Rh negative are called “universal donors” because their red blood cells can be transfused to people of all blood groups. This is why the Red Cross is constantly recruiting donors whose blood type is O Rh negative.

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