How many different ABO blood types exist among humans?

There are four distinct ABO blood types in humans: A, B, AB, and O. This classification is based on the presence or absence of specific antigens on the surface of red blood cells. Blood type A has A antigens, blood type B has B antigens, blood type AB has both A and B antigens, and blood type O has neither. Understanding this system is crucial in fields such as transfusion medicine, organ transplantation, and prenatal care, as compatibility between donor and recipient blood types is essential for the safe administration of blood products. The four blood types also influence immune responses and can have implications for disease susceptibility, making this classification highly significant in both clinical and research settings.