All human blood is made up of basically the same plasma, cells, and other chemical materials. But individuals differ in some of the arrangements and proportions of the chemicals in their cells and plasma.
There are four main groups, or types, of blood, and every human being can be classified under one of them. The groups are called A, B, O, and AB, based on the presence or absence of certain protein molecules in the blood.
When blood from two different groups is mixed and the blood clumps, it is because of a reaction between the protein molecules in the red cells and the plasma. Such chemical reactions make it dangerous for a person to receive a transfusion of whole blood from someone whose blood group is unknown. But if the cells are removed from blood, then the remaining plasma can be given to anyone, no matter what his blood group.
Blood can be exchanged among human beings whose groups and subgroups have been matched. However, certain large populations may have more of one particular group than another. Anthropologists who study man’s physical development use blood groups as one of the ways of showing relationships among individuals and population groups.
So although human blood has many different groups and subgroups, it is basically all the same. In fact, each species of animal has its own kind of blood. For example, all cats have the same kind of blood, just as all dogs have the same kind of blood. The blood of one species cannot be exchanged for the blood of another.