Blood group AB. Computer artwork of red blood cells (erythrocytes) in the shape of the letters A and B. This represents blood group AB. There are four blood groups, A, B, AB and O, each determined by the presence or absence of carbohydrate molecules (antigens) on the surface of red blood cells. There are two types of blood group antigen; A and B. The lack of any antigen gives blood group O. If an incompatible blood type is given during a transfusion it can cause fatal immune reactions.