There are four major blood types: A, B, AB, and 0. The blood types are determined by proteins called antigens (also called agglutinogens) on the surface of the RBC. There are two antigens, A and B. If you have the A antigen on the RBC, then you have type A blood. When B antigen is present, you have type B blood. When both A and B antigens are present, you have type AB blood. When neither are present, you have type O blood.