The term human blood group systems is defined by the International Society of Blood Transfusion as systems in the human species where cell-surface antigens—in particular, those on blood cells—are "controlled at a single gene locus or by two or more very closely linked homologous genes with little or no observable recombination between them", and include the common ABO and Rh antigen systems, as well as many others; 44 human systems are identified as of December 2022. Wikipedia