A blood type comprises the individual composition of proteins on the surface of red blood cells of higher animals, especially the human. The surfaces differ in many proteins that function as antigens. The immune system forms antibodies against foreign antigens. If blood of different blood types is mixed, the cells will clot due to binding antibodies. Before the discovery of blood types, blood transplants had only been successful by chance and usually ended deathly. Blood types are hereditary and thus a feature to prove family relationships, e.g. to prove a father to be wrong. There are about 20 different blood type systems. The AB0-System and the Rhesus-System are the most important ones.