The Kell antigen system (also known as Kell-Cellano system) is a group of antigens on the human re blood cell surface which are important determinants of blood type and are targets for autoimmune or alloimmune diseases which destroy red blood cells. The Kell antigens are peptides found within the kell protein, a 93 kilodalton transmembrane zinc-dependent endopeptidase which is responsible for cleaving endothelin-3.