The spleen is an organ in the upper left side of the abdomen that filters the blood by removing old or damaged blood cells and platelets and helps the immune system by destroying bacteria and other foreign substances. It also holds extra blood that can be released into the circulatory system, if needed.
The spleen is a useful but nonessential organ. It is sometimes removed (splenectomy) in people who have blood disorders, such as thalassemia or hemolytic anemia. If the spleen is removed, a person must get certain immunizations to help prevent infections that the spleen normally fights.