The Rh system is a classification of blood types based on the presence or absence of the Rh factor, a specific protein found on the surface of red blood cells. If a person has this protein, they are considered Rh positive (Rh+), while those who do not have it are classified as Rh negative (Rh-). This system is important for blood transfusions and pregnancy, as incompatible blood types can lead to serious health issues. The Rh factor was discovered in the 1940s through research involving the blood of rhesus monkeys. Understanding the Rh system helps medical professionals ensure safe blood transfusions and manage conditions like hemolytic disease of the newborn, where an Rh- mother may produce antibodies against her Rh+ baby's blood.