On the blood determines a persons blod type

It is determined by the presence or absence of certain identifiers on the surface of the red blood cells. There are four main ABO blood type groupings: A, B, AB, and O. These blood groups are determined by the antigen on the blood cell surface and the antibodies present in the blood plasma.

The test to determine your blood group is called ABO typing. Your blood is mixed with antibodies against type A and B blood. Then the simple is checked to see whether or not the blood cells stick together.