BLOOD ANTIGENS
The immune system normally can tell its own blood cells from other cells. On the surface of red blood cells are proteins that the body sees as antigens. In 1900, a German disease expert (pathologist), Karl Landsteiner, found 2 of these proteins, which he called A and B. Blood is arranged by these antigens into blood types A, B, AB (contains both antigens), and O (contains neither antigen). The liquid part of blood (plasma) has antibodies against the opposite antigen. A person with Type A blood, for example, has antibodies against the B antigen.
In 1940 Dr. Landsteiner found another group of antigens in the blood named Rhesus factors (Rh factors). People with Rhesus factors in their blood are called "Rh positive." Persons without the factors are called "Rh negative." Rh negative people form antibodies against the Rh factor if they are exposed to Rh positive blood.This can be a problem in an Rh negative mother who is pregnant with an Rh positive baby. There are other antigens too, besides ABO and Rh antigens.
