The ABO blood group system classifies human blood into four main types: A, B, AB, and O. These types are determined by the presence or absence of specific antigens on the surface of red blood cells. Type A has A antigens, type B has B antigens, type AB has both, and type O has neither. Blood type is important for blood transfusions and organ transplants, as incompatible blood can cause serious reactions. Each blood type also has corresponding antibodies in the plasma; for example, type A has anti-B antibodies. Understanding these groups helps ensure safe medical procedures.