A chronic disease of the large intestine, also known as the colon, in which the lining of the colon becomes inflamed and develops tiny open sores, or ulcers, that produce pus and mucous. The combination of inflammation and ulceration can cause abdominal discomfort and frequent emptying of the colon. Ulcerative colitis is the result of an abnormal response by your body's immune system. Normally, the cells and proteins that make up the immune system protect you from infection. In people with IBD, however, the immune system mistakes food, bacteria, and other materials in the intestine for foreign or invading substances. When this happens, the body sends white blood cells into the lining of the intestines, where they produce chronic inflammation and ulcerations.