Irritable bowel syndrome (IBS) is the most common condition seen by gastroenterologists. It presents with alternating symptoms of bowel dysfunction that often worsens with stress. The cause of these symptoms eludes investigators and many attempts have been made to discover an underlying pathology. This is a daunting task since symptoms come and go, and change characteristics. Furthermore, the pathology of IBS is unlikely to be identical in all patients. In addition, all symptoms and all features studied thus far have a strong overlap with healthy volunteers. Elsewhere in this issue, Braak et al. report a well-designed clinical investigation in patients with IBS and come to the conclusion that IBS is not characterized by mast cell or other immune cell proliferation, but by immune dysregulation in the colon. Is this the fi nal answer? Am J Gastroenterol 2012; 107:727 -729; doi: 10.1038/ajg.2012 Irritable bowel syndrome (IBS) is a common functional disorder in the West ( 1 ) and in Asia ( 2 ), which is characterized clinically as abdominal discomfort or pain with alternating constipation and diarrhea. Mechanistic pathways to symptom generation are unclear, although dysfunction of brain-gut communication, gut motility, visceral perception, neuroimmune responses, infl ammation, and mucosal integrity, as well as the psychosocial status might be involved ( 3 ). Th e mucosa contains endings of the enteric sensory nerves; these aff erent nerves can modify motility and pain perception. Th erefore, immune cell infi ltration assessed in mucosal biopsies may provide critical information on the mechanisms of symptom generation. Both intestinal ( 4 ) and colonic ( 5,6 ) biopsies have been investigated. Mast cells have received particular attention because its secretory products, such as histamine, prostaglandins, and proteinases, are capable of sensorimotor dysfunction. IBS appears to worsen with stress and the important information might be obtained from relationships between stress, corticotropin-releasing hormone (CRH) and symptoms ( 7 ). CRH signaling pathways have been shown to have an important role in stress-related alterations of gut function related to intestinal symptoms ( 7 ). Braak et al. ( 8 ) have attempted to clarify IBS pathophysiology using these strategies.Mast cells are well known to modulate gastrointestinal pathophysiology and are considered to be involved in IBS ( 9 ). Increased numbers of mast cells have been reported in the terminal ileum in patients with IBS, although there was lack of correlation with diarrhea-predominant IBS symptoms ( 4 ). Braak et al. ( 8 ) conclude that the number of mast cells, macrophages (CD68), T cells (CD3 and CD8), and λ FLC-positive mast cells is decreased in IBS patients compared with healthy volunteers. As a general statement, this needs clarifi cation. Th ey note the decrease in the descending colon, but no change was seen in the ascending colon and immune cell numbers in the small intestine were not measured. Furthermore, mast cells in the gut musculature ( ...