Spleen

a large vascular lymphatic organ lying in the upper part of the abdominal cavity on the left side, between the stomach and diaphragm, composed of white and red pulp

Description
The spleen is a blood-forming organ in early life and later a storage organ for red corpuscles and platelets. Because of a large number of macrophages in the spleen, it also acts as a blood filter, both identifying and destroying defective erythrocytes. The white pulp of the spleen consists of lymphatic nodules and diffuse lymphatic tissue, the red of venous sinusoids between which there are splenic cords. The stroma of both red and white pulp is reticular fibers and cells. A framework of fibroelastic trabeculae extending from the capsule subdivides the structure into poorly defined lobules.

Relevance to Hemoglobinopathies
In blood disorders with an increased turnover of erythrocytes, such as sickle cell disease and the thalassemias, the spleen becomes enlarged owing to its filtration of defective cells for removal and, in the more severe forms of diseases, an expansion of erythroid progenitors owing to extramedullary hematopoiesis. In patients with severe enlargement of the spleen, splenectomy may be indicated, e.g. to reduce the requirement of blood transfusions and to avoid other medical complications.