The spleen is an important organ in the body that has a variety of responsibilities. The spleen Is a major blood filter, which helps remove old and damaged red blood cells  and bacteria. It Is a  part of the lymphatic system and produces lymphocytes. (A type of white blood cell that are a part of the immune system that helps to prevent and fight infection). It acts as a reservoir for red blood cells and platelets, should the body need them. Although the spleen is only about the size of our fist, at any given moment about 4% of all the blood in our body is passing through the spleen.

A Blood Filter. One of the spleen’s most important functions is to filter out microorganisms and foreign particles that have entered our bloodstream. For instance, in experiments, scientists have observed that foreign dye particles discoloring the blood can be entirely removed in one pass through the spleen.

Not only does the spleen remove foreign particles and microorganisms, it also removes worn-out and defective red blood cells. Because of this, the spleen may have to be removed from patients with sickle cell anemia lest the spleen destroy all of the patient’s defective sickle-shaped red blood cells. After all, even bad blood is better than no blood. Thanks be to God that, when the spleen is removed, the liver can take over some of the spleen’s functions.

The removal of defective or worn-out blood cells occurs in certain areas of the spleen called the red pulp (Figure 2), named for the high concentration of red blood cells. Red blood cells normally circulate in our blood vessels for about 120 days before they wear out. During its life span each red blood cell makes many trips through the spleen, where it is subjected to a grueling gauntlet that tests cell function and flexibility. Those blood cells failing to pass the tests are removed from circulation.

Blood passing through the spleen may take either a direct closed route through its blood vessels or an indirect open route where the blood temporarily leaves the blood vessels. (If blood leaves the blood vessels almost anywhere else in our body, it results in massive blood clotting or internal bleeding.) Those blood cells that leave the blood vessels in the spleen’s red pulp must essentially find their own way back into circulation by squeezing through the walls of special blood vessels called sinusoids. This strange arrangement actually tests the flexibility of the red blood cells.