The first rhesus blood type was discovered in 1937 by Landsteiner and Wiener, who named it after a similar factor found in rhesus monkey blood. The significance of the discovery was not immediately apparent and was only realized in 1940, after subsequent findings by Philip Levine and Rufus Stetson.[2] The serum that led to the discovery was produced by immunizing rabbits with red blood cells from a rhesus macaque. The antigen that induced this immunization was designated by them as Rh factor to indicate that rhesus blood had been used for the production of the serum.[3]