There are two antigens, called A and B, that it is possible to have on the surface of your red blood cells. If the cells have only A, it means the blood is of group A, and likewise with B. If both are present then you're AB, and if neither O. It is important to know your blood group, as if you are given the wrong type they can react and cause death. The table shows which groups can, and cannot, be mixed during transfusions.