The ABO Blood Group System is a classification of human blood based on the presence or absence of specific antigens on the surface of red blood cells. There are four main blood types: A, B, AB, and O. Type A has A antigens, type B has B antigens, type AB has both, and type O has neither. This system is crucial for blood transfusions, as incompatible blood types can cause serious reactions. In addition to the antigens, the ABO Blood Group System also involves antibodies in the plasma. For example, a person with type A blood has anti-B antibodies, while a person with type B blood has anti-A antibodies. Type AB individuals have no anti-A or anti-B antibodies, making them universal recipients, while type O individuals have both antibodies, making them universal donors. Understanding this system is essential for safe medical practices.