Ulcerative colitis (UC) is a type of inflammatory bowel disease (IBD) that affects the large intestine.

The large intestine consists of the cecum, ascending/tranverse/descending/sigmoid  colon and rectum. UC usually begins in the rectum and may continue up into the colon. When inflamed, the intestinal lining can become red, swollen, develop ulcers and bleed. This inflammation and irritation can prevent the large intestine from absorbing enough fluid and salt from stool, leading to diarrhea.

UC is a chronic, lifelong disease with alternating periods of activity and remission. Patients may experience intense symptoms followed by varying periods of time when the symptoms seem to disappear.

The cause of UC is unknown, but it appears to involve dysfunction of the  immune system. Protective cells normally present in the gastrointestinal lining are triggered to attack when bacteria and viruses pass through the digestive tract. In patients with UC, this attack continues, even when harmful substances are no longer present – leading to chronic inflammation and irritation. It is not clear whether this immune dysfunction is a cause or result of UC.

UC appears most often in people between the ages of 15 and 35, but may also occur in older or younger populations. About 500,000 Americans have UC, according to the Crohn’s & Colitis Foundation of America (CCFA). Another 500,000 have Crohn’s disease (CD), the other major type of IBD.

UC differs from CD in a number of ways. In patients with UC, only the large intestine is affected whereas CD may occur anywhere throughout the digestive tract, including the small and large intestines. Inflammation caused by UC is usually continuous, without any normal tissue appearing between inflamed areas. In CD, there may be patches of normal tissue between inflamed areas. In addition, UC affects only the inner lining of the intestine, so ulcers are not likely to break through the innermost intestinal wall layer (mucosa), as may occur in patients with CD. 

Inflammatory bowel diseases such as UC and CD are sometimes confused with irritable bowel syndrome (IBS). However, IBS does not involve inflammation of the intestines, which is present in both UC and CD. Also, IBS does not include bloody diarrhea as a symptom and IBS tests reveal no abnormalities. IBS is not associated with either UC or CD.