Definition of Crohn's disease:

Crohn's disease is a form of inflammatory bowel disease (IBD), which involves ongoing (chronic) inflammation of the gastrointestinal tract. Crohn's-related inflammation usually affects the intestines, but may occur anywhere from the mouth to the end of the rectum (anus).

See also: Ulcerative colitis

Causes, incidence, and risk factors:

While the exact cause of Crohn's disease is unknown, the condition is linked to a problem with the body's immune system response.

Normally, the immune system helps protect the body, but with Crohn's disease the immune system can't tell the difference between good substances and foreign invaders. The result is an overactive immune response that leads to chronic inflammation. This is called an autoimmune disorder.

There are five different types of Crohn's disease:

• Ileocolitis is the most common form. It affects the lowest part of the small intestine (ileum) and the large intestine (colon).
• Ileitis affects the ileum.
• Gastroduodenal Crohn's disease causes inflammation in the stomach and first part of the small intestine, called the duodenum.
• Jejunoileitis causes spotty patches of inflammation in the top half of the small intestine (jejunum).
• Crohn's (granulomatous) colitis only affects the large intestine.

A person's genes and environmental factors seem to play a role in the development of Crohn's disease. The body may be overreacting to normal bacteria in the intestines.

The inflammation related to Crohn's disease frequently occurs at the end of the small intestine that joins the large intestine, but it may occur in any area of the digestive tract. There can be healthy patches of tissue between diseased areas. The ongoing inflammation causes the intestinal wall to become thick.

The disease may occur at any age, but it usually occurs in people between ages 15 - 35. Risk factors include:

• Family history of Crohn's disease
• Jewish ancestry
• Smoking