Rh factor is also called “Rhesus factor” because it was first discovered in the blood of Rhesus monkeys (small monkeys from India often used for experimentation).
Rh factor is an antigen, a substance which stimulates the production of antibodies to fight foreign invaders, such as viruses, bacteria and transplanted organs. A given individual either has the antigen already in their blood (they are Rh positive), or they don’t (they are Rh negative). A patient’s Rh status effects how he or she handles blood transfusions or organ transplants.
Prior to the twentieth century, blood and its function was poorly understood. In trying to solve the problem of serious blood loss from injuries, doctors tried to inject (transfuse) blood from another person or animal into the injured patient. In some cases, this worked and the patient recovered. In many more cases, however, the blood transfusion actually harmed the patient, often causing death. No one could predict which type of reaction would occur as a result of a blood transfusion. So, by the beginning of the nineteenth century, most European nations had outlawed the practice of blood transfusion.
About 1900 Austrian-American physician Karl Landsteiner (1868-1943) developed an explanation for the phenomenon of blood rejection. Landsteiner found that human blood serum (the liquid portion of blood surrounding the cells) could be divided into four categories, depending on its ability to cause clotting of red blood cells. He gave these groups the names A, B, AB, and 0 based on what type of clotting antigen they had, if any.
In 1940 Landsteiner discovered another of blood factor antigen, known as Rh. This discovery resulted from Landsteiner’s studies with Rhesus monkeys. Landsteiner and his colleagues found that when blood from monkeys was injected into rabbits and guinea pigs, it clotted. This was because of the presence of another antigen that the researchers had not classified before. Landsteiner called this antigen the Rh (Rhesus) factor. Researchers also showed that the factor occurs among some, but not all, humans. It is also inherited.