346 red blood cell antigens and 33 platelet antigens have been serologically defined. The genetic basis for most of these antigens lie in 45 red blood cell or six platelet genes. Thirty-five major human blood group systems (including the ABO and Rh- systems) were recognised by the International Society of Blood Transfusion (ISBT) in October 2012. In addition to the ABO antigens and Rh (Rhesus) antigens, many other antigens are expressed on the red blood cell surface membrane. For example, an individual can be AB RhD positive, and at the same time M and N positive (MNS system), K positive (Kell system), and Le^a or Le^b positive (Lewis system). Many of the blood group systems were named after the patients in whom the corresponding antibodies were initially encountered.