Of all the organs of the immune system, the spleen is the largest. It is located on the upper left-hand side of the abdomen, in front of the diaphragm and behind the stomach. The size of the spleen can vary considerably but is, on average, about the size of a fist. At any given time, the spleen contains a substantial amount of blood, which it filters as part of its immune system function. It is also, as mentioned, a storage center for leukocytes.
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