Why are there Four Human Blood Types?
Human blood types are inherited and contain genetic contributions from each parent. Blood types are determined by antigens, which are molecules that trigger immune response on the surface of red blood cells. There are actually vastly more than four blood types — the International Society of Blood Transfusion (ISBT) recognizes 29 different blood group systems, encompassing over 600 distinct antigens. However, the most well known and medically useful blood group system, called ABO, distinguishes four blood types.
