“…Serotonin receptors belong to the G-protein-coupled receptor family with exception of the 5-HT 3 receptor, which is an ion channel (Barnes and Sharp, 1999). Despite the major role of 5-HT in the CNS, ∼90% of 5-HT is produced in the gastro-intestinal tract (Racké et al, 1996), where it is involved in the regulation of multiple physiological processes such as stimulation of cytokine and chemokine production (Dürk et al, 2005;Idzko et al, 2004;Müller et al, 2009), cell proliferation (Pakala and Benedict, 1998;Pakala et al, 1997), migration (Kushnir-Sukhov et al, 2006;Müller et al, 2009;Tamura et al, 1997) and the regulation of the immune system (Ahern, 2011). Expression of 5-HT receptors has been identified on a broad range of immune cells, including T cells (O'Connell et al, 2006), macrophages (Mikulski et al, 2010) and dendritic cells (Idzko et al, 2004).…”