Review article: distinctions between ileal and colonic Crohn's disease: from physiology to pathology

Background Ileal and colonic Crohn's disease seem to be two separate entities. Aims To describe the main physiological distinctions between the small and the large intestine and to analyse the differences between ileal and colonic Crohn's disease. Methods The relevant literature was critically examined and synthesised. Results The small and large intestine have fundamental distinctions (anatomy, cellular populations, immune defence, microbiota). The differences between ileal and colonic Crohn's disease are highlighted by a heterogeneous body of evidence including clinical features (natural history of the disease, efficacy of treatments, and monitoring), epidemiological data (smoking status, age, gender) and biological data (genetics, microbiota, immunity, mesenteric fat). However, the contribution of these factors to disease location remains poorly understood. Conclusion The classification of ileal and colonic Crohn's disease as distinct subphenotypes is well supported by the literature. Understanding of these differences could be exploited to develop more individualised patient care.