Dear student,

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).

Rh factor was first discovered in Rhesus Monkey (small monkeys from India often used for experimentation) by Karl Landsteiner and A.S. Weiner in 1940.

In 1940, 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. And from this RH factor came to existence.