The spleen is the largest immunological organ in the body. It lies intraperitoneally on the left side of the abdomen inferior to the diaphragm. Its main function are immune responses and phagocytosis.
The spleen is about 12 cm long and weighs 150 g. It is located in the left hypochondriac region between the ninth and twelfth rib where it is surrounded by a layer of fibroelastic connective tissue. It is almost completely covered by peritoneum except the hilum where the vessels and nerves pass through. A healthy, non-enlarged spleen does not extend beyond the rib cage and, thus, is not palpable. In newborns it has a rather lobulated structure which may persist until adulthood. About 10% of all people have additional splenic tissue, e.g. near the pancreatic tail or along splenic vessels, which is referred to as an accessory spleen.
Anterior to the spleen is the stomach and posteriorly the convexity of the diaphragm. Due its proximity to the costodiaphragmatic recess it is partly covered by the lower border of the lungs. The spleen sits upon the splenic flexure of the colon (the curvature between the transverse and descending colon). Medially it borders with the left kidney. To a small extent it also reaches the pancreatic tail near the hilum.
Three ligaments originating from the surrounding structures attach to the spleen. The gastrosplenic ligament stretches between the greater curvature of the stomach to the hilum of the spleen. The splenorenal ligament connects the left kidney with the spleen at the same point as the latter ligament. Lastly, the spleen sits on the phrenicocolic ligament which originates from the colon and is also known as the sustentaculum lienis.
The splenic artery originates from the celiac trunk and splits at least into two branches before entering the hilum and reaching the splenic lobes. These branch off smaller arteries to the splenic segments (segmental splenic arteries). The splenic vein arises at the hilum and drains venous blood from the segmental splenic veins. It courses behind the pancreatic body towards the hepatic portal vein.
Fibers from the celiac ganglion supply the organ both sympathetically and parasympathetically. The nerves accompany the splenic arteries.
The splenic lymph nodes lie at the hilum and receive lymph via perivascular and subcapsular lymph vessels. It is then drained to the celiac and superior pancreatic lymph nodes.
The spleen is part of the mononuclear phagocyte system (MPS) and innate immune system. It ingests bacteria and other harmful substances. The lymphoid follicles of the white pulp has B cells which produce antibodies. Furthermore T cells are located around the periarteriolar lymphoid sheaths which recognize antigens presented on MHC molecules. About one third of the removal of physiologically aged red blood occurs in the spleen (erythrophagocytosis). To a small degree it also stores erythrocytes and thrombocytes. During the embryological development the spleen is involved in the production of red blood cells (erythropoiesis) until the eighth fetal month.
Splenectomy is the surgical removal of the spleen either in its entirety or partially. Despite its important function for the immune systems the spleen is a non-essential organ. The reasons for removing the spleen include an extensive splenomegaly, rupture, a severe infection, a wandering spleen and certain blood disorders, e.g. sickle cell anemia and immune thrombocytopenic purpura. Patients with a removed spleen are prone to infections and need additional vaccinations and preventive antibiotics (if necessary) for the rest of their lives. Therefore the indication for splenectomy has become stricter in comparison to the past.