System for classifying blood according to presence or absence of the Rh antigen (factor) in erythrocytes .
Rh-negative persons who receive Rh-positive blood transfusions produce antibodies to Rh factor, which attack red blood cells with the factor if they are ever received again, causing serious illness and sometimes death. The antibodies also attack the red cells of an Rh-positive fetus carried by an Rh-negative woman if she has had a previous Rh-positive transfusion or pregnancy (see erythroblastosis fetalis ). The Rh-negative trait is rare worldwide but more common in some ethnic groups. See also blood typing .