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Data obtained from animal and human studies support the existence of mast cells (MC) in the gastrointestinal (GI) tract, where they are considered important for the control of luminal parasites and possibly other microorganisms. MC are also implicated in food allergies, GI inflammation, and functional GI diseases, including irritable bowel syndrome (IBS) [1]. MC density may be increased in inflammatory bowel disease (IBD) and IBS, although such studies have yielded variable and sometimes contradictory results, likely due to methodological problems such as sampling issues, poorly characterized MC identification methodology, and lack of adequate documentation of disease correlates. There is no question that MC numbers are likely to be easier to identify and count in lesional areas as they may be present in IBD. MC density in the terminal ileum had previously been reported to be significantly increased in patients with IBS, even though MC abundance only correlated weakly with IBS symptoms, and, in some cases, a decreased density of colonic c-kit-positive in IgEor IgG-stained MC, was reported [2], even though IgEor IgG-staining is not an index of MC activation. In the paper by Choi et al. [3] published in this issue of Digestive Diseases and Sciences, the authors analyzed intestinal MC density in: (a) diverse patient cohorts, including diarrhea predominant-IBS (D-IBS), ulcerative colitis (UC) in remission, mild UC, and in healthy controls; (b) random colonic biopsies; (c) different colonic segments; and (d) mucosal MC and lamina propria/intra-epithelial T lymphocytes. The results are important because MC number was significantly increased in 97.6 % of D-IBS patients, intra-epithelial lymphocytes (IEL) were increased in 92.8 %, and lamina propria lymphocytes (LPL) were increased in 81.9 %. Moreover, even though the numbers of immune cells were even higher in some post-infectious (PI)-IBS patients, the remaining D-IBS patients had a significantly higher number of these cells compared with controls. Immunochemically identified were tryptase-positive MC and CD3-positive T cells (and not for all lymphocytes, as the authors stated). All publications to date, including the one by Choi et al. [3], did not report MC activation, which, according to some, is more important than actual numbers [4]. For instance, tryptase staining underestimates the number of degranulated MC, since released secretory granule tryptase does not stain. One way to identify degranulated MC is to stain for c-kit (CD117), the surface MC receptor for stem cell factor. Nevertheless, since mediator secretion can also occur without degranulation [5], MC activation can best be identified either by ultrastructural appearance T. C. Theoharides (&) Molecular Immunopharmacology and Drug Discovery Laboratory, Department of Integrative Physiology and Pathobiology, Tufts University School of Medicine, 136 Harrison Avenue, Boston, MA 02111, USA e-mail: theoharis.theoharides@tufts.edu

Mast Cells in Irritable Bowel Syndrome and Ulcerative Colitis: Function Not Numbers Is What Makes All the Difference