A blood type describes the individual composition of proteins on the surface of red blood cells of higher animals, especially humans. The surfaces differ by numerous proteins that serve as antigens. The immune system produces antibodies against foreign antigens. If blood of different blood types is mixed, cells clot due to binding of antibodies. 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. paternal relations. There are around 20 different blood type systems for humans. The Rhesus-System and the AB0-System are the most important.