Key Concepts

• The pathophysiology of IBD is incompletely understood but thought to be multifactorial and a complex interaction between genetic, environmental, microbial, and immune factors.
• While hundreds of susceptibility genetic loci have been identified to increase the risk of disease, all identified loci individually contribute only a small percentage of the expected heritability in IBD.
• The significantly increasing incidence of IBD in developing urban countries worldwide lends strong evidence that environmental factors may play the dominate role in its pathogenesis.
• The composition of the gut microbiota is significantly different in patients with IBD and has increased colonization of Fusobacterium and members of the Proteobacteria phylum and decreased colonization of Firmicutes and Bacteroidetes. Yet, no single organism has been identified to be solely responsible for the development of IBD.