The spleen is an organ found in virtually all vertebrate animals with important roles in regards to red blood cells and the immune system Spleen, Internet Encyclopedia of Science In humans, it's located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve in case of hemorrhagic shock, especially in animals like horses (not in humans), while recycling iron.Mebius RE, Kraal G. (2005). Structure and function of the spleen. Nat Rev Immunol. 5(8):606-16. PMID… (More on Spleen)