The immune system normally can distinguish its own blood cells from other cells. The surface of red blood cells contain several proteins that can be identified by the body as antigens.
Blood plasma contains antibodies against the opposite antigen.
A person with Type A blood, for example, has antibodies against the B antigen. In 1940, Dr. Landsteiner discovered another group of antigens. Rh negative persons form antibodies against the Rh factor if they are exposed to Rh positive blood. This is of major importance in an Rh negative mother who is pregnant with an Rh positive baby. There are other antigens as well, besides ABO and Rh antigens.