Rh factor describes blood type compatibility issues involving Rh blood groups. These play a critical role in transfusions and in obstetrics.

Rh blood groups were first discovered in 1940 by Karl Landsteiner and Alexander Wiener in their experiments with Rhesus monkeys, hence the name. Forty years earlier, Landsteiner had identified blood types A, B, AB and O, a discovery that made blood transfusions safe.

Landsteiner and Wiener noted that unexplained accidents in transfusions were attributed to this Rh blood group. The Rh blood group is also a correlary in hemolytic disease of the newborn, which is characterized by the breaking apart of red blood cells.

People with the Rh (rhesus) protein on the surface of their red blood cells are called Rh-positive (about 85 % of the population). Those who lack the protein are Rh-negative (about 15 %). The immune system of an Rh-negative person destroys Rh-positive blood, as it does...