If blood is in short supply, which blood type would be the most beneficial to have on hand if someone needed a blood transfusion?
1.o+
2.o–
3.ab+
4.ab–

Answer is O⁻.

The human blood group is determined by the type of antigens (A and B) present or absent on the surface of the red blood cells and antibodies in the plasma. Besides antigen A and B another antigen Rh is present or absent on the surface of red blood cells.

For examples blood group A contains A antigen on the red blood cells and anti-B antibody in the plasma. Further blood group A may be A⁺ or A⁻. if Rh antigen is present then it is A⁺ and if absent A⁻.  Blood group O does not contains either A or B antigens.  Rh may be present or absent.

Blood group O⁻ blood can be given to people with any blood type. That's because it has none of the antigens and Rh antigen.  People with this blood type are considered "universal donors" and are in high demand at blood banks.

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