The spleen plays important roles in regard to red blood cells (erythrocytes) and the immune system.

As a part of the mononuclear phagocyte system, it metabolizes hemoglobin removed from senescent red blood cells.

[5][6][7] The spleen is a center of activity of the mononuclear phagocyte system and is analogous to a large lymph node, as its absence causes a predisposition to certain infections.

The gastric surface is directed forward, upward, and toward the middle, is broad and concave, and is in contact with the posterior wall of the stomach.

The underlying central nervous processes coordinating the spleen's function seem to be embedded into the hypothalamic-pituitary-adrenal-axis, and the brainstem, especially the subfornical organ.

A spleen easily palpable below the costal margin in any child over the age of three to four years should be considered abnormal until proven otherwise.

Trauma, such as a road traffic collision, can cause rupture of the spleen, which is a situation requiring immediate medical attention.

Asplenia refers to a non-functioning spleen, which may be congenital, or caused by traumatic injury, surgical resection (splenectomy) or a disease such as sickle cell anaemia.

Splenosis is a condition where displaced pieces of splenic tissue (often following trauma or splenectomy) autotransplant in the abdominal cavity as accessory spleens.

[33][34] There has been a long and varied history of misconceptions regarding the physiological role of the spleen, and it has often been seen as a reservoir for juices closely linked to digestion.

[35] The spleen also plays an important role in traditional Chinese medicine, where it is considered to be a key organ that displays the Yin aspect of the Earth element (its Yang counterpart is the stomach).

Etymologically, spleen comes from the Ancient Greek σπλήν (splḗn), where it was the idiomatic equivalent of the heart in modern English.

[38] In 1701, Anne Finch (later, Countess of Winchilsea) had published a Pindaric ode, The Spleen, drawing on her first-hand experiences of an affliction which, at the time, also had a reputation of being a fashionably upper-class disease of the English.

[39] Both Blackmore and George Cheyne treated this malady as the male equivalent of "the vapours", while preferring the more learned terms "hypochondriasis" and "hysteria".

[38][40][41] In the late 18th century, the German word Spleen came to denote eccentric and hypochondriac tendencies that were thought to be characteristic of English people.

This usage was popularised by the poems of Charles Baudelaire (1821–1867) and his collection Le Spleen de Paris, but it was also present in earlier 19th-century Romantic literature.

[43] In cartilaginous and ray-finned fish, the spleen consists primarily of red pulp and is normally somewhat elongated, as it lies inside the serosal lining of the intestine.

In most vertebrates, the spleen continues to produce red blood cells throughout life; only in mammals this function is lost in middle-aged adults.

In cetaceans and manatees, they tend to be quite small, but in deep diving pinnipeds, they can be massive, due to their function of storing red blood cells.

Even in these animals, there is a diffuse layer of haematopoeitic tissue within the gut wall, which has a similar structure to red pulp and is presumed homologous with the spleen of higher vertebrates.

[44] In mice, the spleen stores half the body's monocytes so that, upon injury, they can migrate to the injured tissue and transform into dendritic cells and macrophages to assist wound healing.