Spleen

The spleen is the largest lymphatic organ in the body. It is located in the upper left quadrant of the abdominal cavity just beneath the diaphragm, and is normally protected by the lower rib cage.

The spleen is a soft purplish color and somewhat flattened, measuring about 12.5 to 16 cm long (5-6 inches) and 5 to 7 cm (2-3 inches) wide. Despite its size, the shape and structure of the spleen is similar to other lymph nodes. It filters blood, rather than lymph, at a rate of about 4% of the total in circulation per minute.

There are two types of tissue in the spleen

white pulp (lymphatic tissue consisting mainly of lymphocytes surrounding arteries)
red pulp (contains venous sinuses filled with such blood and disease-preventing cells as lymphocytes and macrophages)

Blood enters the spleen through the splenic artery and cleansed as it flows through the organ. Leukocytes within the spleen trap destroy microorganisms. The cleansed blood leaves via the splenic vein.

The spleen is made up of:

B-cells
T-cells
macrophages
dendritic cells
natural killer cells
red blood cells

In addition to capturing foreign materials (antigens) from the blood that pass through it, the spleen also receives antigens via migratory macrophages and dendritic cells that bring them to the spleen via the bloodstream. An immune response is initiated when the macrophage or dendritic cells present the antigen to the appropriate B or T cells, which produce large amounts of antibodies.

Functions

filters and cleanses
a significant part of the immune system response.
acts as a reservoir or storage area for blood, supplying it in an emergency, as a hemorrhage. At such a time, the muscles in the spleen contract, forcing the stored blood out and back into general circulation.
destroys and phagocytoses old, worn-out blood cells. If the spleen becomes too aggressive, blood cells can be removed from circulation prematurely, causing a low red blood cell count (anemia) and a low platelet count (thrombocytopenia).
assists in red blood cell production (erythropoiesis) during pregnancy.

Damage

Because the spleen tears so easily, it is often injured during accidents or contact sports and has to be removed. The organ is almost impossible to repair because of its fine network and the huge amount of blood it is able to store, considering its size. Therefore, injury to the spleen will often cause prolonged bleeding, necessitating removal. This is called a splenectomy.

The spleen can also be damaged through infection, inflammation, or invasion by cancer cells. Because it is the largest filtering organ of the lymphatic system, removal dramatically increases the risk for infection. Seemingly harmless infections can prove fatal to those who have had to have their spleens removed. Since the liver and bone marrow provide essentially the same service, there is no noticeable change when the spleen is removed UNTIL there is an overload of microbes. Then, if the liver and bone marrow cannot handle the onslaught, a serious infection develops. The most dangerous of the organisms to those who have undergone a splenectomy is the pneumococcus bacterium. This microbe can multiply so rapidly in the blood that it produces shock in a matter of hours. Less common, but equally fatal, are the bacteria Hemophilus and Neisseria.

This page was updated in December 2005.