Human blood is typed according to whether certain markers, called antigens, are present on the surface of red blood cells. The ABO system classifies blood as type A, B, AB, or O based on the presence or absence of the A and B antigens; the Rh system classifies blood as negative or positive based on the presence or absence of the Rh antigen (Rh factor).

If red blood cells have:

• The A antigen, they are type A blood.
• The B antigen, they are type B blood.
• Both the A and B antigens, they are type AB blood.
• Neither A nor B antigens, they are type O blood.

Rh blood typing determines the presence (+) or absence (–) of the Rh factor. If red blood cells:

• Have the Rh antigen, they are Rh-positive.
• Do not have the Rh antigen, they are Rh-negative.