The ABO and Rh blood types are determined by antigens residing on the surface of the red blood cell. These antigens will cause an antigen-antibody reaction (see Immunity) when they come in contact with the antibodies against them. Individuals with the A or B antigens automatically possess antibodies to the antigen they don't have (see below). For this reason you can only transfuse blood into a recipient who does not carry antibodies to the cells being transfused.
| Blood Type | Antigens Present | Antibodies Present in Plasma | Approx. % of Population |
| A | A | anti B | 41 |
| B | B | anti A | 10 |
| O | neither A nor B | both anti-B and Anti-A | 45 |
| AB | both A and B | neither | 4 |
The Rh factor (named for its discovery in Rhesus monkeys) is one of several antigens, C, D, or E. Since D is the most common it is the one usually tested. If you have the D antigen you are Rh positive, if you lack it you are Rh negative. 85% of the population is Rh positive. One does not possess the anti-Rh antibodies unless he or she is exposed to Rh antigens. This could occur accidentally through transfusion, and for women it occurs when the placental blood vessels break down at or near delivery. In the case of transfusion it is important to use the same ABO and Rh type blood, or use O negative, sometimes called the universal donor. If a mother who is Rh negative is giving birth to an Rh positive baby she will be given a drug to suppress the immune system to avoid producing the anti-Rh antibodies.