There are four major blood groups determined by the presence or absence of two antigens A and B on the surface of red blood cells: Group A has only the A antigen on red cells (and B antibody in the plasma) Group B has only the B antigen on red cells (and A antibody in the plasma) Group AB has both A and B antigens on red cells (but neither A nor B antibody in the plasma) Group O has neither A nor B antigens on red cells (but both A and B antibody are in the plasma). In addition to the A and B antigens, there is a third antigen called the Rh factor, which can be either present (+) or absent. In general, Rh negative blood is given to Rh-negative patients, and Rh positive blood or Rh negative blood may be given to Rh positive patients. The universal red cell donor has Type O negative blood type. The universal plasma donor has Type AB positive blood type.