The spleen is involved in the immune system and is responsible for removing old blood cells.  It sits under the rib cage in humans on the left side of the abdomen.  In the cat, the spleen is an elongated structure that is often reddish-brown in color on preserved specimens.   A splenectomy is is a surgical procedure where the spleen is removed, often because of an abdominal injury that ruptures the organ.  A person who has had their spleen removed is more likely get get illnesses or infections because of a decrease in immune function.