Blood groups are antigenic determinants on the surface of blood cells, encoded either by a single gene or by a cluster of closely linked, homologous genes. There are 30 blood group systems, some of which contain only one determinant while others contain many. The genes controlling blood group systems have been cloned and sequenced and located on specific chromosomes. There are additional blood group antigens, mostly of very high or very low prevalence, which have not been assigned to a system due to insufficient genetical evidence. Antibodies to blood groups are usually formed in response to antigen-positive red cells as the result of transfusion or pregnancy. Some of these antibodies are clinically relevant in terms of transfusion reactions and hemolytic disease of the fetus and newborn. The structure and function of most components carrying blood group antigens have been elucidated.