The role of serotonin and its receptors in activation of immune responses and inflammation

Serotonin or 5‐hydroxytryptamine (5‐HT) is a neurotransmitter and hormone that contributes to the regulation of various physiological functions by its actions in the central nervous system (CNS) and in the respective organ systems. Peripheral 5‐HT is predominantly produced by enterochromaffin (EC) cells of the gastrointestinal (GI) tract. These gut‐resident cells produce much more 5‐HT than all neuronal and other sources combined, establishing EC cells as the main source of this biogenic amine in the human body. Peripheral 5‐HT is also a potent immune modulator and affects various immune cells through its receptors and via the recently identified process of serotonylation. Alterations in 5‐HT signalling have been described in inflammatory conditions of the gut, such as inflammatory bowel disease. The association between 5‐HT and inflammation, however, is not limited to the gut, as changes in 5‐HT levels have also been reported in patients with allergic airway inflammation and rheumatoid arthritis. Based on searches for terms such as ‘5‐HT’, ‘EC cell’, ‘immune cells’ and ‘inflammation’ in pubmed.gov as well as by utilizing pertinent reviews, the current review aims to provide an update on the role of 5‐HT in biological functions with a particular focus on immune activation and inflammation.