The ABO blood group system. There are four blood types within the ABO system. The blood type is determined by the presence or the absence of two different antigens on the surface of the red blood cells. Antigens A and B. Our molecules are called agglutinogens. How do we get four blood types with only two antigens? If the red blood cells carry agglutinogens A, then the blood type is A. If the red blood cells carry agglutinogens B, then the blood type is B. If the red blood cells carry both agglutinogens A and B, then the blood type is AB. If the red blood cells carry no agglutinogens, then the blood type is O. So the four blood types are A, B, AB, and O. The blood is made up of blood cells, red and white, platelets, and a liquid called plasma. The plasma contains many types of molecules including antibodies, which are also called agglutinogens. Blood type A has agglutinogens anti-B. Blood type B has agglutinogens anti-A. Blood type AB has no agglutinogens. Blood type O has agglutinogens anti-A and anti-B. The following table shows the blood groups and their corresponding agglutinogens and agglutinogens. Agglutination is the clumping of RBCs by agglutinogens. When agglutinogens and agglutinogens correspond, agglutination happens. For each blood type, the agglutinogens in the plasma do not correspond to the antigens on the surface of the RBC, otherwise agglutination occurs.