The Rhesus factor is an inherited protein found on the surface of red blood cells. Most people have this protein and are called Rh-positive. However, some people don't have this protein; they are called Rh-negative. Rh-negative pregnant women are at risk of having a baby with a potentially dangerous form of anemia called Rh disease if they have a baby with an RH-positive man but, treatment can prevent Rh disease.
Attribution: Straight Talk Foundation