The ABO System is a classification system for human blood types 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. Blood type is determined by the combination of these antigens, which can affect blood transfusions and organ transplants. In addition to the A and B antigens, the ABO System also considers the Rh factor, which indicates whether the blood type is positive or negative. Understanding blood types is crucial for medical procedures, as incompatible blood transfusions can lead to serious health complications.