Environmental, genetic, infectious, and host factors have all been implicated as a cause of both Crohn's disease and ulcerative colitis. Among the environmental factors, smoking has been associated with an increased recurrence rate of Crohn's disease. Mycobacterium paratuberculosis and the measles virus have received recent attention and have also been considered as possible etiologies of Crohn's disease. There are few data to support a primary causative role of psychogenic factors. Immunologic factors have received greatest attention. Several mechanisms of injury have been proposed, including autoimmune destruction of the gut mucosal cells as the result of cross-reactivity with antigens from enteric bacteria as well as nonspecific immunologic injury to the gut mucosa as the result of a chronic inflammatory process for both ulcerative colitis and Crohn's disease. Cytokines, including interleukins and tumor necrosis factor, have been invoked in the perpetuation of the inflammatory response. Whether immune factors play a primary or secondary role in the pathogenesis of these diseases is not known. Extraintestinal manifestations suggest a role for immune complexes or an autoantibody response at various involved sites.
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