The function of the spleen, a lymphatic organ located in the abdominal cavity, was not known until fairly recently. Though the spleen and the "black bile" it secretes have had various meanings in the folk medicine of various cultures since ancient times, its actual functions remained a mystery until the mid-20th century. In humans, the spleen consists of two different tissue types — red pulp and white pulp — with two distinct functions.
In Ancient Greece, the spleen was believed to produce black bile or melancholy, one of the "four humours" that had to be balanced in the body for proper functioning. An excess of black bile was considered responsible for melancholia, a condition that is currently recognized as clinical depression. In traditional Chinese medicine, the spleen is said to influence one's temperament and willpower.
Today, the spleen is more completely understood. Red pulp, also called splenic pulp, consists of blood and reticular fibers. This portion of the human spleen helps to filter the blood, purging it of unwanted elements which contain red blood cells that have aged to the point of deterioration. White pulp, also called Malpighian bodies of the spleen or splenic lymphoid nodules, refers to small nodules within the spleen that are rich in lymphocytes and help to fight infection.
The spleen is a very important organ, and asplenia, a condition in which the spleen is not present either congenitally or due to removal in surgery, has been linked to an increased predisposition to certain infections. The spleen also serves as an emergency reservoir of blood. In some animals, it stores red blood cells, while in humans, it stores platelets, the clotting agent in blood.
The spleen also has a few minor functions. In the human fetus, the spleen is a secondary site of red blood cell production until the fifth month of gestation, though after birth, red blood cells are only produced in the bone marrow. Other metabolic products, however, are produced in the spleen throughout adulthood; namely, opsonins, properdin, and tuftsin.