page_title: Spleen
section_title: Other animals
hierarchical_section_title: Spleen / Other animals
caption_attribution_description: Deutsch: Laparoskopische Ansicht der Milz eines Pferdes (grün/blaues Organ). English: Laparoscopic view of a horse's spleen (The purple and grey mottled organ).
context_page_description: The spleen is an organ found in virtually all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes from Ancient Greek σπλήν.
  The spleen plays important roles in regard to red blood cells 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 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 synthesizes antibodies in its white pulp and removes antibody-coated bacteria and antibody-coated blood cells by way of blood and lymph node circulation. These monocytes, upon moving to injured tissue, turn into dendritic cells and macrophages while promoting tissue healing. 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.
context_section_description: In cartilaginous and ray-finned fish, it consists primarily of red pulp and is normally somewhat elongated, as it lies inside the serosal lining of the intestine. In many amphibians, especially frogs, it has the more rounded form and there is often a greater quantity of white pulp. A study published in 2009 using mice found that the red pulp of the spleen forms a reservoir that contains over half of the body's monocytes.
  In reptiles, birds, and mammals, white pulp is always relatively plentiful, and in birds and mammals the spleen is typically rounded, but it adjusts its shape somewhat to the arrangement of the surrounding organs.  In most vertebrates, the spleen continues to produce red blood cells throughout life; only in mammals this function is lost in middle-aged adults. Many mammals have tiny spleen-like structures known as haemal nodes throughout the body that are presumed to have the same function as the spleen. The spleens of aquatic mammals differ in some ways from those of fully land-dwelling mammals; in general they are bluish in colour. In cetaceans and manatees they tend to be quite small, but in deep diving pinnipeds, they can be massive, due to their function of storing red blood cells.
  The only vertebrates lacking a spleen are the lampreys and hagfishes (the Cyclostomata). Even in these animals, there is a diffuse layer of haematopoeitic tissue within the gut wall, which has a similar structure to red pulp and is presumed homologous with the spleen of higher vertebrates.
  In mice the spleen stores half the body's monocytes so that upon injury, they can migrate to the injured tissue and transform into dendritic cells and macrophages to assist wound healing.
caption: Laparoscopic view of a horse's spleen (the purple and grey mottled organ)
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