Besides the RhD (RH1) antigen, other important antigens of the Rh system (ISBT, number 004) are: C (RH2), E (RH3), c (RH4) and e (RH5). Appropriate antigen-positive red cells may stimulate antibody production in antigen C (RH2), c (RH4), E (RH3) and e (RH5) negative individuals. The determination of the Rh phenotypes can, therefore be important during pregnancy, for previously transfused patients and for patients with known irregular antibodies. Approximately 9% of the Caucasian population are K (KEL1, K, K1) positive. The K antigen is strongly immunogenic. Anti-K has been reported as the cause of hemolytic transfusion reactions, both immediate and delayed, and hemolytic disease of the newborn.