The spleen is the largest distinct organ of the lymphatic system. Similar in structure to a large lymph node, it acts primarily as a blood filter. Despite this important function, a healthy life is possible after removal. The spleen plays important roles in regards to red blood cells and the immune system. 

Structure of the Spleen

The spleen is located in the left upper quadrant of the abdomen. It is similar to an enlarged lymph node but is a bit more complex. The spleen is made up of two distinct tissue types:

• Red pulp is the site of blood filtration in the spleen. It is made of connective tissue called the cord of Billroth that can fill with blood and contains many macrophages.
• White pulp is secondary lymphoid tissue that is similar to that in the adenoid tonsils. They contain large amounts of lymphocytes and antigen-presenting cells.

Unlike lymph nodes, the spleen possesses only efferent lymphatic vessels, because it only filters blood instead of lymph fluid. The splenic artery forms its primary blood supply. The spleen is unique in respect to its development within the gut because it is derived from mesenchymal tissue rather than endoderm tissue during embryonic development. However, it still shares the same blood supply as the foregut organs in the abdominal cavity.

Spleen

This diagram of the spleen indicates the vein, artery, white pulp, red pulp, and capsule.

Function of the Spleen

The primary function of the spleen is blood filtration. Blood cells have a lifespan of roughly 120 days. When blood passes through the red pulp of the spleen, healthy blood cells easily pass, while older red blood cells are caught phagocytized by the macrophages within. The macrophages also remove pathogens, denatured hemoglobin, and other cellular debris. Iron from old or damaged hemoglobin content in the blood is filtered out and sent to the liver so that new red blood cells can be created. Antigens are also filtered by the red pulp, which may be presented to naive lymphocytes in the white pulp of the spleen. This stimulates the same type of adaptive immune response that occurs in the lymph nodes. 

The spleen is also important for generating new red blood cells early in embryonic development, but this function stops after birth. The spleen may also function as a reservoir of blood and platelets during hypovolemic shock, which occurs when overall tissue perfusion falls due to severe dehydration or severe bleeding or hemorrhage. During hypovolemic shock, the spleen can release up to a cup of extra blood to help mitigate the complications of fluid loss.

The spleen is often removed surgically if it becomes damaged or infected. This causes modest increases in circulating white blood cells and platelets, diminished responsiveness to some vaccines, and increased susceptibility to infection by bacteria and protozoa. In particular, there is an increased risk to infection from gram negative bacteria that cause pneumonia. Besides these increased risks, the loss of the spleen does not cause major immune system impairment, and most people will still live normal and healthy lives because the lymph nodes and liver perform the same functions as the spleen. In particular, those with splenomegaly (an enlarged spleen that could rupture) or splenic cancers are typically better off living without their spleen than living with the risk of severe bleeding from a ruptured spleen or the plethora of symptoms caused by splenic cancer and its metastases.