- an inherited antigen found on the red blood cells of most people and certain other primates: those with this antigen are Rh positive while those lacking it are Rh negative , the determination of which is especially important when assessing blood-type compatibility for transfusions and pregnancy.
- an agglutinogen commonly found in human blood: it may cause a haemolytic reaction, esp during pregnancy or following transfusion of blood that does not contain this agglutinogen
