The spleen is a small, fist-sized organ located in the upper left side of the abdomen. It plays a vital role in the body's immune system by filtering blood and helping to fight infections. The spleen also stores red blood cells and platelets, which are important for carrying oxygen and helping with blood clotting. In addition to its immune functions, the spleen helps recycle iron from old red blood cells, making it essential for maintaining healthy blood levels. If the spleen is damaged or removed, other organs like the liver can take over some of its functions, but the body may be more vulnerable to infections.