The spleen is an organ located in the upper left part of the abdomen, behind the stomach. It plays a crucial role in the immune system by filtering blood and removing old or damaged red blood cells. The spleen also helps produce white blood cells, which are essential for fighting infections. In addition to its immune functions, the spleen stores blood and helps regulate blood volume. It can release stored blood into the circulatory system when needed, such as during physical activity or injury. Overall, the spleen is vital for maintaining a healthy blood supply and supporting the body's defense mechanisms.