Ulcerative colitis involves only the colon; Crohn's disease can involve either the colon, or the small bowel, or both. In ulcerative colitis, there are mucosal ulcers and infiltration of the mucosa and submucosa with neutrophils, macrophages, and lymphocytes. In Crohn's disease, the inflammatory infiltrate frequently contains granulomas and extends through all layers of the bowel wall rather than being confined to the mucosa and submucosa as in ulcerative colitis. Both diseases have been viewed as “chronic” inflammatory diseases because of their prolonged clinical courses and because their inflammatory infiltrates contain lymphocytes and macrophages, a histologic picture that is characteristic of chronic inflammation. Although the etiologic agents are unknown, recent studies have given some insights into the mechanism for the amplification of the inflammatory response that result in the histologic and clinical changes characteristic of these diseases. It is common thought that a response to a lumenal antigen would result in more uniform inflammation, as is seen in gluten-sensitive enteropa The limitation to the intestine may reflect a defect in the regulation of the mucosal immune system but not the general immune system. The immune response seen in IBD is determined by the characteristics of the intestinal immune system, which is a unique functional immmunologic compartment in comparison with peripheral blood MNC.