HRH4 and inflammatory bowel disease: As discussed above, mast cells, allocated to the innate immune system, do not only produce histamine, but also respond to stimulation of the H4R. Besides the contribution of mast cells to allergic inflammation [132], they also seem to be involved in non-allergic inflammatory diseases of the gut, since in the intestine of IBD patients, parallel to mucosal histamine concentrations, number and activity of mast cells are increased [133,134].