A blood type describes the individual composition of proteins on the surface of red blood cells of higher animals, especially the human. The surfaces differ in numerous proteins that serve as antigens. The immune system produces antibodies against foreign antigens. If blood of different types is mixed, clotting of the cells due to binding of antibodies occurs. Before the discovery of blood types, blood transfusions were only successful by chance and usually ended deadly. Blood types are hereditary and therefore a feature, to prove relations, e.g. for paternity tests. There are approximately 20 different blood type systems in the human. The AB0-System and the Rhesus-System are the most important ones.