Rh factor is a substance on the red blood cells of most people. Red blood cells that contain the Rh factor agglutinate (clump) if they come into contact with an antibody called anti-Rh. This reaction can produce serious illness or death. People who have the Rh factor are known as Rh-positive. Those lacking it are Rh-negative. Karl Landsteiner, Philip Levine, and Alexander Wiener discovered the factor in rhesus monkeys in 1940. They named it Rh for the monkey.