Enlarged Spleen
(Splenomegaly)

The spleen is an organ located in the upper left quadrant of the abdomen just below the diaphragm and protected under the lower left ribs.

The spleen has a couple of important functions involving blood cells within the body.

1. It filters blood and removes old and damaged red blood cells, bacteria, and other particles as they pass through the maze of blood vessels within the spleen.
   
   
2. It produces lymphocytes, a type of white blood cell that produces antibodies and assists immune system.

The filtration system is part of the red pulp while the white pulp of the spleen contains the immune functioning cells

Normally, the spleen is a small organ about the size of a small fist or orange. Splenomegaly describes the situation where the spleen enlarges in size (spleen+megaly=enlargement).

The spleen enlarges if it is asked to do excessive work in filtering or manufacturing blood cells, if there is abnormal blood flow to it, or if it is invaded with abnormal cells or deposits.

Abnormal red blood cells: Since the spleen filters abnormal blood cells and removes them from the circulatory system, diseases that result in abnormal red cells will cause the spleen to enlarge. Sickle cell disease, thalassemia, and spherocytosis are examples of diseases that form unusually shaped cells that cannot easily maneuver through the small blood vessels and capillaries of the body. If they are not removed by the spleen, these abnormal cells can cause blood clots and decrease circulation. However, removing them causes the spleen to swell and enlarge.

Viral and bacterial infection: The spleen is involved in making cells that fight infection and part of that response is to enlarge. This is commonly seen in viral infections such as infectious mononucleosis (caused by the Epstein Barr virus), AIDS and viral hepatitis. Examples of bacterial infections associated with splenomegaly include tuberculosis, malaria, and anaplasmosis (formerly known as ehrlichiosis).

Splenic vein pressure/blockage: Blood enters the spleen through the splenic artery and leaves through the splenic vein. If the pressure within the vein increases or if the splenic vein becomes blocked, blood cannot leave the spleen and it may swell. Because of the relationship to liver blood flow, cirrhosis and portal vein obstruction can cause complications with venous blood flow from the spleen. Congestive heart failure may cause both the liver and spleen to swell because of increased venous pressure.

Cancers: Leukemias and both Hodgkins and non-Hodgkins lymphoma can cause the spleen to enlarge, as can a variety of other tumors including melanomas.

Metabolic disease: Metabolic diseases that enlarge the spleen include Niemann-Pick disease, Gaucher's disease, and Hurler syndrome.

Source: MedicineNet.com
http://www.medicinenet.com/enlarged_spleen_splenomegaly/article.htm