In short, Crohn's disease is a chronic disorder which causes inflammation of the digestive or gastrointestinal (GI) tract. Although it can affect any area of the GI tract, it most commonly affects the small intestine and/or colon.
Because the symptoms of Crohn's disease and ulcerative colitis (UC) are so similar, it is sometimes difficult to establish the diagnosis definitively. In fact, about 10% of colitis cases are called "indeterminate colitis" because they are unable to be pinpointed as either Crohn's disease or UC. Some sufferers can even go months or years before they get the correct diagnosis!
However, Crohn's and UC do have one strong feature in common. Both illnesses are marked by an abnormal response by the body's immune system. Normally, the immune system protects the body from infection. In people with Crohn's disease, however, the immune system reacts inappropriately. Researchers believe that the immune system mistakes bacteria that is normally found lining the walls of the intestine for foreign or invading substances, and launches an attack. In the process, the body sends white blood cells into the lining of the intestines, where they produce chronic inflammation. These cells then generate harmful products that ultimately lead to ulcerations and bowel injury. When this happens, the patient experiences the symptoms commonly associated with Crohn's disease, including such fun things as diarrhea, bleeding, abdominal pain, and subsequent malnutrition and weight loss among others.
Healthy small bowel on the left, inflamed small bowel on the right in a patient with Crohn's. Yup, the one on the right hurts.
Although Crohn's disease most commonly affects the end of the small intestine (the ileum) and the beginning of the large intestine (the colon), it may involve any part of the GI tract. In Crohn's disease, all layers of the intestinal tissue may be involved, and there can be normal healthy bowel in between patches of diseased bowel. If inflamed bowel does not react appropriately to drug therapies, the patient may have to have a portion of the affected intestine removed surgically. Furthermore, Crohn's disease patients are also at a significantly higher risk for colon cancer, and must have a colonoscopy every two years or so to check for cancerous polyps that may develop in the colon.