Spleen

The spleen (from Anglo-Norman espleen; ult. from Ancient Greek σπλήν, splḗn), or milt, is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The spleen plays important roles in regard to red blood cells (erythrocytes) and the immune system. It removes old red blood cells and holds a reserve of blood, which can be valuable in case of hemorrhagic shock, and also recycles iron. As a part of the mononuclear phagocyte system, it metabolizes hemoglobin removed from senescent red blood cells. The spleen is a center of activity of the mononuclear phagocyte system and is analogous to a large lymph node, as its absence causes a predisposition to certain infections. The globin portion of hemoglobin is degraded to its constitutive amino acids, and the heme portion is metabolized to bilirubin, which is removed in the liver. The spleen contains red and white pulp, which have distinct purposes contributing to adaptive and innate immunity. The white pulp houses T lymphocytes in periarteriolar lymphoid sheaths (PALS) situated around central arterioles and B lymphocytes in follicles adjacent to PALS. This positioning allows the B lymphocytes to encounter blood-borne antigens, which activate B cells to differentiate into plasma cells. Plasma cells then secrete antibodies, such as IgM antibodies and later IgG antibodies. Antigen-specific responses by B cells exemplify the spleen's role in the adaptive immune system. The red pulp surrounds the white pulp and marginal zone, consisting of venous sinuses and splenic cords called the cords of Billroth. Blood traveling through the red pulp is "filtered" as it encounters actively phagocytic macrophages, which remove old or damaged red blood cells as well as other antibody-coated cells and pathogens. The red pulp also stores monocytes, which can be mobilized to travel to...