The presence of a protein in the blood plasma
The differentiation among blood groups results from the presence of a certain type of protein on the surface of red blood cells in the body. This protein or antigen has two types which are named Antigen A and Antigen B. A person whose blood group is A has the antigen A on the surface his red blood cells, while a person whose blood group is B has the antigen B on the surface of his red blood cells. However, a person who has neither A nor B antigen has the blood group O, while a person who has both A and B antigen has the blood group AB.