The spleen is a fist-sized, spongy organ located behind the stomach (removed in this illustration), just under the diaphragm. Part of the lymphatic system, the spleen is composed of two types of tissue. White pulp produces lymphocytes, which release antibodies into the bloodstream to combat infection. More prominent is the red pulp, in which macrophages filter broken cells, parasites, bile pigments, and other useless substances from the blood. Iron from broken red blood cells is stored in the spleen for later use. Despite this variety of functions, an injured or diseased spleen can be removed in adults without great consequence.

 Spleen, flattened, oblong organ that removes disease-producing organisms and worn-out red blood cells from the bloodstream. The spleen is situated in the upper left abdominal cavity, in contact with the pancreas, the diaphragm, and the left kidney. It is supported by bands of fibers that are attached to the peritoneum (the membrane lining the abdominal cavity). The spleen varies in size but, in adult humans, it is about 13 cm (about 5 in) long, up to 10 cm (4 in) wide and 3.8 cm (1.5 in) thick, and weighs about 200 g (about 7 oz). The spleen is fed by the splenic artery, and venous blood from the spleen enters the liver.

 The spleen removes iron from the hemoglobin of red blood cells for use in the body. It also removes such waste materials as bile pigments for excretion as bile by the liver. The spleen produces antibodies against various disease organisms and manufactures a variety of blood cells. In some mammals (but not in humans) it stores red blood cells and feeds them into the circulation to maintain the volume of blood in cases of hemorrhage. In the unborn child, the organ functions primarily to produce red blood cells, a function that is normally taken over by the bone marrow after birth. However, if diseases slow down this function in the bone marrow, the spleen may again start to produce red blood cells.

 Many diseases affect the spleen. In splenomegaly, the spleen enlarges. This condition is usually an indication of bacterial, parasitic, and viral infections, such as infectious mononucleosis, tuberculosis, malaria, rheumatoid arthritis, and histoplasmosis. Splenomegaly may also be caused by cirrhosis of the liver. In hypersplenism, a malfunctioning or hyperactive spleen may so increase its normal rate of destroying red blood cells as to cause a variety of hemolytic anemias. Hemorrhage of an enlarged spleen may magnify anemia and also cause an iron deficiency. Small splenic abscesses may run a benign course without detection. Large splenic abscesses, however, may rupture into the bowel cavity to cause peritonitis (inflammation of the abdominal wall lining), and the infection may invade the bloodstream and be carried to the liver, where it may form additional abscesses. Splenic cysts are rare, and the spleen is seldom the site of a carcinoma, or cancer.

 A hyperactive spleen may sometimes be treated with radiation or corticosteroids (steroids that are extracted from the adrenal cortex). Abscesses are treated with broad-spectrum antibiotics or surgical drainage. Surgical removal of the spleen is required if the spleen or the splenic artery ruptures.