Spleen enlargement is an enlargement of the spleen beyond its normal size. The spleen is located on the left side of the abdomen and weighs around 200g (8oz) in the average healthy adult. The spleen can be considered a dual-purpose organ: it filters the blood and removes abnormal cells (such as old and defective red blood cells), and it makes disease-fighting components of the immune system (including antibodies and lymphocytes). Since the spleen is involved in so many bodily functions, it is vulnerable to a wide range of disorders involving the blood or lymph system, and by infection, malignancies, liver disease, or parasites.

The body of the spleen appears red and pulpy, surrounded by a tough capsule. The red pulp consists of blood vessels (splenic sinusoids) interwoven with connective tissue (splenic cords).  The red pulp filters the blood and removes old and defective blood cells. The white pulp is inside the red pulp, and consists of little lumps of lymphoid tissue. Antibodies are made inside the white pulp. Similar to other organs of the lymphatic system, particular immune cells and blood cells are either made or matured inside the spleen.  Blood enters the spleen via the splenic artery, which subdivides into many tiny branches. Each branch is encased in a clump of lymphocytes, which means every drop of blood is filtered for foreign particles as it enters the spleen.

Hypersplenism is the name given to the condition where the spleen becomes overactive and destroys more blood cells than it should. Symptoms depend on which blood component is lacking. For example, if red blood cells are deficient, anaemia will result (with symptoms including fatigue and pallor). Most cases of hypersplenism are caused by disorders somewhere else in the body, such as Cirrhosis of the liver.

Splenic rupture can be caused by certain disorders such as infectious mononucleosis: the spleen becomes delicate enough to spontaneously rupture.  A sudden blow to the abdomen can split the outer capsule of the spleen and cause bleeding into the abdominal cavity. There are various degrees of splenic rupture. When bleeding is life-threatening, surgery to remove the spleen (splenectomy) is needed.

Causes of Enlarged Spleen
Splenomegaly occurs in about 10% of systemic lupus erythematosus patients. A variety of disorders can cause the spleen to enlarge, sometimes to 2kg (roughly 4lbs) or more.  Any condition that causes a rapid breakdown of blood cells, such as haemolytic anaemia, can place great strain on the spleen and make it enlarge. This includes:

Hemolytic anemias
Thalassemia
Hemoglobinopathies
Hemolytic Anemia due to G6PD deficiency
Idiopathic autoimmune hemolytic anemia
Immune hemolytic anemia