How many blood types are there in the ABO blood group system?

There are four blood types in the ABO blood group system, which are: 1. Type A blood: Only agglutinogen A is present on red blood cells, and the serum contains anti-B agglutinin. 2. Type B blood: Only agglutinogen B is present on red blood cells, and the serum contains anti-A agglutinin. 3. Type AB blood: Both agglutinogens A and B are present on red blood cells, and the serum contains no anti-A or anti-B agglutinin. 4. Type O blood: Neither agglutinogen A nor B is present on red blood cells, and the serum contains both anti-A and anti-B agglutinin. These four blood types are distinguished based on the presence of different agglutinogens on the surface of red blood cells. Each blood type has unique genetic characteristics and clinical significance in blood transfusion.