The blood typing system is a method of categorizing blood based on the presence or absence of certain antigens. Antigens are molecules that can trigger an immune response in the body, and they are found on the surface of red blood cells. The most common blood typing system is the ABO system, which is based on the presence or absence of two antigens, A and B. Blood types are classified as either A, B, AB, or O, depending on the presence or absence of these antigens.