Introduction

The spleen, a spongy, soft organ about as big as a person's fist, is located in the upper left part of the abdomen, just under the rib cage. The splenic artery brings blood to the spleen from the heart. Blood leaves the spleen through the splenic vein, which drains into a larger vein (the portal vein) that carries the blood to the liver. The spleen has a covering of fibrous tissue (the splenic capsule) that supports its blood vessels and lymphatic vessels.

See the figure Viewing the Spleen.

The spleen is made up of two basic types of tissue: the white pulp and the red pulp, each with different functions. The white pulp is part of the infection-fighting (immune) system. It produces white blood cells called lymphocytes, which in turn produce antibodies (specialized proteins that protect against invasion by a foreign substance). The red pulp filters the blood, removing unwanted material. The red pulp contains other white blood cells called phagocytes that ingest microorganisms, such as bacteria, fungi, and viruses. It also monitors red blood cells, destroying those that are abnormal or too old or damaged to function properly. In addition, the red pulp serves as a reservoir for different elements of the blood, especially white blood cells and platelets (cell-like particles involved in clotting). However, releasing these elements is a minor function of the red pulp.

A person can live without a spleen. Sometimes the spleen must be removed surgically (splenectomy) because of irreparable damage (for example, due to an injury sustained in a car accident). When the spleen is removed, the body loses some of its ability to produce protective antibodies and to remove unwanted microorganisms from the blood. As a result, the body's ability to fight infections is impaired. People who do not have a spleen are at particularly high risk of pneumococcal infections because of the spleen's role in fighting certain kinds of bacteria, such as pneumococcus. Despite these problems, however, the spleen is not critical to survival: Other organs (primarily the liver) compensate for the loss by increasing their infection-fighting ability and by monitoring for and removing red blood cells that are abnormal, too old, or damaged.