The spleen, in healthy adult humans, is approximately 11 centimetres (4.3 in) in length. It usually weighs between 150 grams (5.3 oz) and 200 grams (7.1 oz) and lies beneath the 9th to the 12th thoracic ribs.
The spleen is part of the lymphatic system.
The spleen is unique in respect to its development within the gut. While most of the gut viscera are endodermally derived (with the exception of the neural-crest derived suprarenal gland), the spleen is derived from mesenchymal tissue. Specifically, the spleen forms within, and from, the dorsal mesentery. However, it still shares the same blood supply — the celiac trunk — as the foregut organs.
|red pulp||Mechanical filtration of red blood cells. In mice: Reserve of monocytes|
|white pulp||Active immune response through humoral and cell-mediated pathways.||Composed of nodules, called Malpighian corpuscles. These are composed of:|
Other functions of the spleen are less prominent, especially in the healthy adult:
- Production of opsonins, properdin, and tuftsin.
- Creation of red blood cells. While the bone marrow is the primary site of hematopoiesis in the adult, the spleen has important hematopoietic functions up until the fifth month of gestation. After birth, erythropoietic functions cease, except in some hematologic disorders. As a major lymphoid organ and a central player in the reticuloendothelial system, the spleen retains the ability to produce lymphocytes and, as such, remains an hematopoietic organ.
- Storage of red blood cells and other formed elements. In horses roughly 30% of the red blood cells are stored there. The red blood cells can be released when needed. In humans, up to a cup (236.5ml) of red blood cells can be held in the spleen and released in cases of hypovolemia. It can also store platelets in case of an emergency.
- 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 and so assist wound healing.
 Effect of removal
Surgical removal causes:
- modest increases in circulating white blood cells and platelets,
- diminished responsiveness to some vaccines,
- increased susceptibility to infection by bacteria and protozoa; in particular, there is an increased risk of sepsis from polysaccharide encapsulated bacteria.
A 28-year follow-up of 740 veterans of World War II who had their spleen removed on the battlefield found that those who had been splenectomised showed a significant excess of mortality from pneumonia (6 rather than the expected 1.3) and a significant excess of mortality from ischaemic heart disease (4.1 rather than the expected 3) but not from other conditions.source: http://en.wikipedia.org/wiki/Spleen
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AWESOME ARTICLE BY NEW YORK TIMES: FINALLY THE SPLEEN GETS SOME RESPECT
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