Immune activation and gut microbes in irritable bowel syndrome

Irritable bowel syndrome (IBS) is a highly prevalent disorder that is characterized by chronic abdominal pain and altered bowel habit. The diagnosis of IBS has traditionally been made by matching the complaints of the patient with established clinical criteria, since the underlying pathophysiology was not known. Various new findings have recently been reported in IBS patients that challenge our concept of IBS as a syndrome with no explanation. While the florid inflammation characteristic of inflammatory bowel disease is absent in IBS, changes suggesting immune activation are present in nearly all IBS patients. Is IBS an autoimmune disease, or is the immune activation responding to a trigger? In this review we present evidence that points to a state of immune activation in IBS and show data that suggest that small intestinal bacterial overgrowth triggers immune activation in IBS.