SPLEEN - A solid organ situated in the upper left-hand side of the abdomen just under the ribcage. The spleen functions to store red blood cells and remove "old" red blood cells from the circulation. It also plays a role as part of the reticuloendothelial system, that helps fight infections in the body. Although not essential to life, it does provide increased resistance to infection by encapsulated bacteria (e.g. Pneumococcus).