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White pulp is the part of the spleen where lymphocytes are maintained in a similar way as in lymph nodes.
Survival is possible without a spleen. However, retrospective epidemiological studies of a veterans veterans of World War II who had their spleens removed on the battlefield found that those who had been splenectomized showed a significant excess of mortality risk from pneumonia and a significant excess of mortality from ischemic heart disease, but not from other conditions.
The spleen is the largest distinct organ of the lymphatic system. Similar in structure to a large lymph node, the spleen acts primarily as a blood filter. Despite this important function, a healthy life is possible after removal. The spleen plays important roles in regards to red blood cells and the immune system.
Structure of the Spleen
The spleen is located in the left upper quadrant of the abdomen. It seems similar to an enlarged lymph node, but is a bit more complex. The spleen is made up of two distinct tissue types:
Red Pulp:the site of blood filtration in the spleen. It is made of connective tissue called the cord of Billroth that can fill with blood, which contains a large amount of macrophages.
White Pulp: secondary lymphoid tissue that is similar to that in the adenoid tonsils. They contain large amounts of lymphocytes and antigen presenting cells.
Unlike lymph nodes, the spleen possesses only efferent lymphatic vessels, because it only filters blood instead of lymph fluid. The splenic artery forms its primary blood supply. The spleen is unique in respect to its development within the gut because it is dervied from the mesenchymal tissue rather than the endoderm tissue during embryonic development. However, it still shares the same blood supply as the foregut organs in the abdominal cavity.
The primary function of the spleen is blood filtration. Blood cells have a lifespan of roughly 120 days, and stop functioning after that. When blood passes through the red pulp of the spleen, healthy blood cells will easily pass while, older red blood cells will be caught and the macrophages within will phagocytize them. The macrophages also remove pathogens, denatured hemoglobin, and other cellular debris. Any iron from old or damaged hemoglobin content in the blood is filtered out as well and sent to the liver, so that new red blood cells can be created. Antigens are also filtered by the red pulp, which may be presented to naive lymphocytes in the white pulp of the spleen, which will stimulate the same type of adaptive immune response that occurs in the lymph nodes.
The spleen is also important for generating new red blood cells early in embryonic development, however this function stops after birth. The spleen may also function as a reservoir of blood and platelets during hypovolemic shock, which occurs when overall tissue perfusion falls due to severe dehydration, or severe bleeding or hemorrhage. During hypovolemic shock, the spleen can release up to a cup of extra blood, to help mitigate the complications of fluid loss for a bit longer.
The spleen is often removed surgically if it becomes damaged or infected. This causes modest increases in circulating white blood cells and platelets, diminished responsiveness to some vaccines, and increased susceptibility to infection by bacteria and protozoa. In particular, there is an increased risk to infection from gram negative bacteria that cause pnuemonias. Besides these increased risks, the loss of the spleen does not cause major immune system impairment, and most people will still live normal and healthy lives because the lymph nodes and liver perform the same functions as the spleen. In particular, those with splenomegaly (an enlarged spleen that could rupture) or splenic cancers are typically better off living without their spleen than living with the risk of severe bleeding from a ruptured spleen or the plethora of symptoms caused by splenic cancer and its metastases.