Animals and bacteria have cell surface antigens referred to as a blood type. Antigens from the human ABO blood group system are also found in apes and old world monkeys, which have inherited the same system. Other animal blood sometimes agglutinates (to varying levels of intensity) with human blood group reagents, but the structure of the blood group antigens in animals is not always identical to those typically found in humans. The classification of most animal blood groups therefore uses different blood typing systems to those used for classification of human blood.