- Omalizumab is a humanized monoclonal antibody that binds to the high-affinity type-I IgE Fc receptors on mast cells (MCs) and basophils, inhibiting the IgE immune pathway. Irritable bowel syndrome (IBS) is the most common functional gastrointestinal disorder, and dysregulation of the immune system likely contributes to its etiology and/or symptomatology. Colonic biopsies from patients with IBS demonstrate considerable increase in the number of degranulating MCs releasing histamine in proximity to nerves, and this event may underlie the common IBS symptom of abdominal pain. Pharmacologic control of MC activation and mediator release is a current area of active interest in the field of IBS research. Recently, we and Pearson et al described 2 cases of patients with IBS with diarrhea (IBS-D) showing positive clinical response to omalizumab. In both cases, the female patients had severe, long-lasting IBS-D and achieved an almost complete resolution of IBS symptoms. Both patients were also able to discontinue all IBS medications after commencing the anti-IgE therapy. For both patients, the omalizumab treatment showed a relatively rapid onset of action, resembling the efficacy observed in and previously reported for patients with chronic spontaneous urticaria. In this Editorial, we discuss the possible biological mechanisms that may underlie the clinical efficacy of omalizumab in IBS. We suggest that there is a need for a well-designed prospective study to investigate the therapeutic effects of anti-IgE in IBS.