A fist-sized blood filter tucked under the left ribs
The spleen, or milt, sits near the liver in almost all vertebrates and works like a large lymph node that filters blood. It culls old red cells, stores reserve blood, recycles iron, and stages immune responses in red and white pulp before a splenectomy becomes necessary.
In people the spleen is a purple organ in the upper left of the abdomen, tucked under the diaphragm behind the ninth to eleventh ribs. A healthy adult's measures about 7 to 14 cm and weighs anywhere from 30 to 230 grams, depending on height, weight and how engorged it is rather than on age or sex. The splenic artery and vein enter at a central slit called the hilum, and four ligaments hold it in place. Unusually for an abdominal organ, it grows not from the gut lining but from mesenchyme in the dorsal mesentery.
Inside are two kinds of tissue. Red pulp, made of venous sinuses and the cords of Billroth, works as the filter: macrophages there engulf worn-out or damaged red cells, antibody-coated cells and microbes, and the spleen breaks down their haemoglobin and recycles the iron. White pulp is immune territory, with T cells sheathed around small arteries and B cells in nearby follicles, where blood-borne antigens can trigger them to become antibody-producing plasma cells. It is the body's largest single mass of lymphoid tissue.
The spleen also banks blood. A horse's holds around 30 percent of its red cells, and a human's keeps up to about a cup, some 240 ml, ready to release when blood volume or oxygen falls. Before birth it makes all types of blood cell, and it helps produce red cells until around the fifth month of gestation, after which the bone marrow takes over.
People can live without it, but at a cost. Removal sharply reduces memory B cells, and a 28-year study of 740 Second World War veterans who lost their spleens in battle found raised death rates from pneumonia and from heart disease. A missing or weak spleen dulls responses to some vaccines and raises infection risk. Enlargement can signal infections such as malaria or glandular fever, blood cancers or storage diseases, and road accidents can rupture it, a medical emergency. About one person in ten has a small accessory spleen, usually around a centimetre across.
Source: Spleen
