Human red blood cells contain protein molecules (antigens) in their cell membranes that determine the blood type of an individual. There are several kinds of antigens present on human red blood cells, as well as the Rh antigen. People with the Rh antigen are distinguished with a blood type ending in a plus (+); those without the Rh antigen have a minus (-) in their blood type.

Rh disease occurs when an Rh-negative mother is exposed to Rh-positive fetal blood and develops antibodies. During pregnancy, and especially during labor and delivery, some of the fetus's Rh-positive red blood cells get into the mother's (Rh -) bloodstream. Higher passage of fetal cells is observed in women who have undergone amniocentesis and other invasive diagnostic procedures, and in women with placental anomalies. This triggering of the mother's immune response is referred to as sensitization, or...