In inflammatory bowel disease, including Crohn's disease and ulcerative colitis, an excessive immune response due primarily to T-cell lymphocytes causes inflammation in the gastrointestinal tract. Lesions in Crohn's disease can occur anywhere in the gastrointestinal tract, i.e., from the oral cavity to the anus. Endoscopically, aphthoid lesions/ulcers believed to be initial lesions progress to discrete ulcers, which coalesce to form a longitudinal array and progress to longitudinal ulcers with a cobblestone appearance, which is a typical endoscopic finding. Before long, complications such as strictures, fistulas, and abscesses form. Lesions in ulcerative colitis generally extend continuously from the rectum and diffusely from a portion of the colon to the entire colon. Endoscopically, lack of vascular pattern, fine granular mucosa, erythema, aphthae, and small yellowish spots are seen in mild cases; coarse mucosa, erosions, small ulcers, bleeding (contact bleeding), and adhesion of mucous, bloody, and purulent discharge in moderate cases; and widespread ulcers and marked spontaneous bleeding in severe cases.