There are five main Rh antigens on red cells for which individuals can be positive or negative: C/c, D and E/e. RhD is the most important in clinical practice. Around 85% of white Northern Europeans are RhD positive, rising to virtually 100% of people of Chinese origin. Antibodies to RhD (anti-D) are only present in RhD negative individuals who have been transfused with RhD positive red cells or in RhD negative women who have been pregnant with an RhD positive baby. IgG anti-D antibodies can cause acute or delayed haemolytic transfusion reactions when RhD positive red cells are transfused and may cause haemolytic disease of the fetus and newborn (HDFN – see Chapter 9). It is important to avoid exposing RhD negative girls and women of child-bearing potential to RhD positive red cell transfusions except in extreme emergencies when no other group is immediately available.