The Rh system belongs along with the AB0 system to the clinically most important systems of blood groups. The RhD antigen is the most immunogenic structure of the red blood cell membrane. The Rh system contains the largest amount of antigens of all blood group systems. Recent years provided many new findings on its biochemical and molecular genetic background. Antigens of the Rh system are on Rh proteins, which are part of the Rh complex of the erythrocyte membrane. There are two very similar Rh proteins – RhD and RhCcEe, coded by two related genes – RHD and RHCE. While RhD positivity implies the presence of RhD protein and RhD negativity its absence, the majority of the other antigens of the Rh system is determined by much smaller differences of individual Rh proteins (e.g. the allelic pair E/e differs only in one amino acid). Rh proteins form in the membrane tetrameric complexes with Rh glycoprotein. In the phenotype and clinical syndrome Rhnull the formation of these tetramers is impossible due to mutations of the gene for Rh glycoprotein or Rh protein and on the red blood cell no Rh antigens are found. Other mutations causing incomplete blocking of complex formation are the cause of poorly expressed Rh antigens. If mutations and reconstructions of RH genes cause a change in the immunogenic part of Rh proteins, Rh variants develop which are characterized by the absence of certain epitopes and the possibility to form anti-D antibodies against normal Rh antigens. At present we differentiate by means of monoclonal antibodies more than 30 RhD epitopes, at the molecular level already more than 30 alleles for D variants were described and 20 alleles for weak D antigens. Key words: Rh system, Rh complex, RHD, RHCE and RHAG genes, RhD and RhCcEe proteins, RhAG glycoprotein, Rh variants, weak antigens

        Key words: Rh system, Rh complex, RHD, RHCE and RHAG genes, RhD and RhCcEe proteins, RhAG