The Rh (Rhesus) blood group system is the second most important blood group system after ABO. It classifies people as either Rh-positive (D antigen present on red blood cells) or Rh-negative (D antigen absent). Approximately 85% of Australians are Rh-positive; 15% are Rh-negative.

Rh compatibility is essential in blood transfusion — giving Rh-positive blood to an Rh-negative recipient can trigger antibody formation. The most important clinical context is pregnancy: if an Rh-negative mother carries an Rh-positive baby, her immune system may produce anti-D antibodies that can cross the placenta in subsequent pregnancies and cause haemolytic disease of the fetus and newborn (HDFN). Anti-D immunoglobulin (Rh immunoglobulin) is given prophylactically to prevent this.