The spleen is an organ located in the upper left side of the abdomen. It is part of the lymphatic system and plays a role in filtering the blood, removing old or damaged red blood cells, and producing immune cells. The liver and pancreas are also organs in the abdomen, but they have different functions. The liver is responsible for detoxification, metabolism, and production of bile, while the pancreas produces digestive enzymes and hormones such as insulin. Therefore, the correct answer is spleen, as it is the only organ listed that is specifically associated with the lymphatic system.