Inflammatory bowel disease: dysfunction of GALT and gut bacterial flora (I).

Gut-associated lymphoid tissue (GALT) is the largest lymphoid organ in the body. This is not surprising considering the huge load of antigens (Ags) from food and commensal bacteria with which it interacts on a daily basis. Gut-associated lymphoid tissue has to recognise and allow the transfer of beneficial Ags whilst concurrently dealing with and successfully removing putative and overtly harmful Ags. This distinctive biological feature of GALT is believed to be crucial to good health. Deregulation or dysfunction of GALT is thought to predispose to inflammatory bowel diseases (IBD) such as ulcerative colitis and Crohn's disease. The exact mechanism(s) underlying the pathogenesis of IBD is (are) poorly understood and the immunological defects in GALT are poorly documented. Advances in immunology have highlighted the importance of dendritic cells (DCs), which are the key Ag presenting cells in tissues and lymphoid compartments. Their crucial role in GALT, in health and disease is discussed in this review. Interaction of DCs with T cells in the gut produces a subset of T lymphocytes, which have immunosuppressive function. Inappropriate Ag uptake and presentation to naïve T cells in mesenteric lymph nodes may lead to T cell tolerance in GALT. These various complex factors in the gut are discussed and their possible relevance to IBD evaluated.