Inflammatory bowel disease (IBD) is among the most common diseases of gastrointestinal system and includes two chronic relapsing diseases: ulcerative colitis and Crohn's disease. These diseases are the result of dysregulated immune response to the gastrointestinal bacteria in people with genetic background (1). There are clinical and microscopic differences between ulcerative colitis and Crohn's disease in terms of pattern of intestinal involvement and involvement of layers of bowel. Although these differences make the differentiation of the two mentioned diseases possible, there is also gray zone; meanwhile, indeterminate colitis (recently known as IBD of undetermined etiology, IBDU) includes 10-15% of the cases. Ulcerative colitis generally starts in the rectum and spreads continuously to the proximal areas. Inflammation is limited to mucosa and sub- mucosa and diffuse and uniform involvement of the affected area. The characteristic histological finding for this disease is defined as destruction and irregularity of colon crypts. Crohn's disease affects most of the gastrointestinal system from the mouth to the anus. The characteristic for this disease is the existence of skip areas and transmural involvement. Granuloma, fissure ulcer and several lymphoid follicles are evident in histology. There may be three factors leading to ulcerative colitis and Crohn's disease: - Genetic background, - Immune response, that is not able to down- regulate
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