Immunological aspects of inflammatory bowel disease

There has been much effort in attempting to incriminate immunological mechanisms in the pathogenesis of inflammatory bowel disease. In ulcerative colitis, although there is continuing interest in the possibility that some cases are due to a type I allergic reaction, possibly to a dietary antigen, the evidence is circumstantial. An abnormal immune response to endogenous bacteria, perhaps perpetuated by cross reactivity with a colonic antigen and mediated by an Arthus type reaction in the mucosa, remains a possibility. Interpretation of reports of lymphocyte cytotoxicity for colonic epithelial cells in ulcerative colitis and impaired cell mediated immunity in Crohn's disease are difficult to interpret. An abnormal immune response to a commonly occurring infective agent is suggested by the result of recent studies on lymphocytotoxins in man and disease transmissibility in animals, but unless this agent is isolated it will not be possible to understand the immunopathological mechanisms involved. It is still not clear whether ulcerative colitis and Crohn's disease are distinct entities of differing aetiology or whether they represent parts of a spectrum of inflammatory bowel disease. Irrespective of the aetiology of inflammatory bowel disease, certain extra intestinal manifestations may prove to be due to an immune response to antigens absorbed from the gut, possibly on the basis of immune complex injury.