A group of antigens on the surface of red blood cells present to a variable degree in human populations. When the Rh factor (an antigen often called D) is present, an individual’s blood type is designated Rh+ (Rh positive); when the Rh antigen is absent, the blood type is Rh- (Rh negative). If an individual with Rh- blood receives a transfusion of Rh+ blood, anti-Rh antibodies form. Subsequent transfusions of Rh blood may result in serious transfusion reactions (agglutination and hemolysis of red blood cells). A pregnant woman who is Rh- may become sensitized by entry of red blood cells from an Rh+ fetus into the maternal circulation after abortion, ectopic pregnancy, or delivery. In subsequent pregnancies, if the fetus is Rh+, Rh antibodies produced in maternal blood may cross the placenta and destroy fetal cells, causing erythroblastosis fetalis.