The spleen is a fist-sized organ situated high up on the left side of the abdominal cavity, behind the stomach. It is composed of two kinds of tissue; one consists of multiple blood-filled passageways, and the other, of lymphoid tissue. One of its functions is to cleanse the blood, much as the lymph nodes cleanse the lymph. Large numbers of phagocytes located in the spleen remove aging or damaged erythrocytes, bacteria, and other foreign materials from the blood. Another function, carried on by the lymphoid tissue, is to provide an immune response to microbial invaders. A third function is to produce new blood cells in rare situations where the bone marrow is unable to meet the demand. The spleen enlarges in a number of infectious diseases, such as infectious mononucleosis and malaria, in which its immune and filtering functions are challenged.
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