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[Gr. splen]
A dark red, oval lymphoid organ in the upper left abdominal quadrant posterior and slightly inferior to the stomach; on the inferior side is the hilum, an indentation at which the splenic vessels and nerves enter or exit. The spleen is surrounded by an outer capsule of connective tissue from which strands of connective tissue (trabeculae) extend into the soft pulp (functional tissue), dividing the spleen into compartments.
The white pulp, composed of lymphocytes and follicles, forms sheaths around arterial vessels and collects in larger nodules containing germinal centers. The red pulp contains vascular sinuses and sinusoids with highly permeable walls, and spongelike splenic cords filled with macrophages and dendritic cells. The spleen is part of the mononuclear phagocytic system and its removal (splenectomy), though compensated for by the lymph nodes and liver, decreases immune function and may place the patient at increased risk for infection, esp. from Streptococcus pneumoniae and Haemophilus influenzae.

In the embryo, the spleen forms both red and white blood cells; after birth, only lymphocytes are created except in severe anemia, when production of red blood cells may be reactivated. Blood enters via the splenic artery and passes through progressively smaller arterial vessels; foreign antigens are trapped in the white pulp, initiating proliferation of antigen-specific lymphocytes and antibodies. The arterioles terminate in the red pulp, where macrophages remove cell debris, microorganisms, and cells that are old, damaged, abnormal, or coated with antibody.

The vascular capacity of the spleen, 100 mL to 300 mL, is an average of 4% of the total blood, and the spleen may contain 30% of the total platelets. In stressful situations, sympathetic impulses stimulate constriction of the venous sinuses, forcing most of the splenic blood into circulation. If the spleen is enlarged (splenomegaly), its vascular capacity increases dramatically, and increased contact with macrophages may cause anemia, leukopenia, and thrombocytopenia. Removal of the spleen may be necessary in patients with thrombocytopenia. Many disorders cause splenomegaly, including portal hypertension, e.g., in cirrhosis, heart failure, and certain infections. Primary disorders of the spleen, however, are rare.
SEE: lymphatic system for illus.; SEE: asplenia syndrome; SEE: germinal center

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