The liver belongs to the digestive system and is the largest substantial digestive organ in the body. Liver cells secrete bile, thereby facilitating the digestion and absorption of fats in the small intestine. The liver is also part of the immune system. Liver cells contain a large number of macrophages, which can play a role in engulfing toxins and removing bacteria. Liver cells also produce antibodies that kill various pathogens that invade the body. The liver also synthesizes a variety of proteins needed by the body, as well as fibrinogen, prothrombin and a variety of coagulation factors related to clotting, such as II, VII, IX and X. In addition, the liver has detoxification functions as well as endocrine functions.