When Wiener and Landsteiner discovered the Rh factor in 1937 (named after the Rhesus monkeys used as test subjects), they did not immediately realize its significance. It was seen as yet another factor, not much different from the M, N, or P factors—useful for "fingerprinting," but not having much more extended implications. However, Wiener soon realized that the new blood factor they had discovered was associated with problems in blood transfusions. Although the first time Rh positive blood is transfused into someone with Rh negative blood, it may not cause any harm, it does cause the creation of antibodies which make a second such transfusion very dangerous. By the time he and Landsteiner published in 1940, Wiener was able to demonstrate the role of Rh sensitization as a cause of intragroup hemolytic reactions, thus increasing the safety of blood transfusions.