This article reviews information regarding the clinical significance of antibodies to antigens in the blood group collections, the 700 series of low-incidence antigens, and the 901 series of high-incidence antigens. Antibodies to many of the antigens in these groups are rarely encountered, meaning that available information is limited. For a few, the clinical significance-the potential to cause reduced survival of transfused antigen-positive red blood cells, a hemolytic transfusion reaction (e.g., anti-AnWj, anti-Emm), or hemolytic disease of the fetus and newborn (e.g., anti-Kg, anti-HJK)-has been documented. Many other specificities have so far been benign (e.g., anti-Csa, anti-M1).