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)

There are very specific ways in which blood types must be matched for a safe transfusion.

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 blood type.