The Reticuloendothelial System (RES) consists of phagocytic cells that remove dead or abnormal cells, tissues, and foreign substances from the body. The RES is made up of monocytes that descend into tissues and become macrophages, with 90% located in the liver. It functions to cleanse the body and is regulated by the nervous system and chemicals in the body, and brain activity influences macrophage activity in the reticular endothelium.