The ABO 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. Each type is determined by the combination of these antigens, which can affect blood transfusions and organ transplants. In addition to the antigens, the ABO system also considers the presence of the Rh factor, which can be either positive or negative. This results in eight possible blood types: A+, A-, B+, B-, AB+, AB-, O+, and O-. Understanding these blood types is crucial for medical procedures and ensuring compatibility between donors and recipients.