The 'Rh' in blood types stands for the Rhesus monkey, which helped scientists discover the Rh factor in 1940.

In 1940, Nobel laureate Karl Landsteiner and his colleague Alexander Wiener conducted experiments at the Rockefeller Institute for Medical Research. They injected red blood cells from the Rhesus macaque (Macaca mulatta) into rabbits and guinea pigs. This process triggered the production of an antibody that agglutinated, or clumped, the red cells of about 85 percent of the human population tested in New York.The researchers realized that these individuals possessed a specific antigen on their red blood cells that was identical to the one found in the monkeys. They named this the Rh factor to honor the species used in the study. This discovery was monumental because it solved the mystery of erythroblastosis fetalis, a condition where a mother's antibodies destroy her fetus's red blood cells. Prior to this, many second-born children died from unexplained hemolytic disease.By the 1960s, researchers developed Rh immunoglobulin, commonly known as RhoGAM, to prevent this immune response. Modern medicine now uses this treatment to ensure compatibility between Rh-negative mothers and Rh-positive fetuses. Interestingly, later research showed that the human Rh factor is not chemically identical to the monkey's factor, leading to the human system being renamed the 'LW' system in some technical circles, though 'Rh' remains the standard clinical term.