Spleen

The spleen is an organ found in almost all vertebrates, similar in structure to a large lymph node, acting primarily as a blood filter. It plays important roles in regard to red blood cells and the immune system, removing old red blood cells, holding a reserve of blood, and recycling iron. As part of the mononuclear phagocyte system, the spleen metabolizes hemoglobin from aged red blood cells; the globin portion is broken down into amino acids, while the heme is processed into bilirubin for removal by the liver. The organ contains two main tissue types: red pulp and white pulp. The white pulp houses T lymphocytes in periarteriolar lymphoid sheaths around central arterioles and B lymphocytes in adjacent follicles, allowing B cells to encounter blood-borne antigens and differentiate into plasma cells that secrete antibodies, including IgM and later IgG. The red pulp surrounds the white pulp and marginal zone, consisting of venous sinuses and splenic cords (cords of Billroth). Here, blood is filtered by phagocytic macrophages that remove old or damaged red blood cells, antibody-coated cells, and pathogens. The red pulp also stores monocytes, which can be mobilized to sites of tissue injury, such as the heart after a myocardial infarction, where they differentiate into dendritic cells and macrophages to promote healing. In humans, the spleen is purple, located in the left upper quadrant of the abdomen beneath the diaphragm and behind the ninth, tenth, and eleventh ribs. It has a smooth convex surface facing the diaphragm and a ridge dividing it into an anterior gastric portion and a posterior renal portion. The gastric surface contacts the posterior stomach wall and the tail of the pancreas, while the renal surface contacts the left kidney and occasionally the left adrenal gland. Four ligaments attach the spleen: gastrosplenic, splenorenal, colicosplenic, and phrenocolic. The hilum, a long fissure near the middle, is the attachment point for the gastrosplenic ligament and entry for the splenic artery and vein, as well as lymphatic vessels and nerves. The spleen is supplied by the splenic artery and short gastric arteries, and is innervated by the splenic plexus, which connects the celiac ganglia to the vagus nerve. During fetal life, the spleen produces all types of blood cells, and it also produces opsonins, properdin, and tuftsin. Surgical removal of the splee