Microscopic Colitis
(Lymphocytic Colitis and Collagenous Colitis)

Colitis means inflammation of the colon. The colon, also known as the large intestine or large bowel, constitutes the last part of the digestive tract. The colon is a long, muscular tube that receives undigested food from the small intestine. It removes water from the undigested food, stores the undigested food, and then eliminates it from the body through bowel movements. The rectum is the last part of the colon adjacent to the anus. The common symptoms of colitis include:

• abdominal pain,
  
  
• diarrhea, and
  
  
• sometimes, rectal bleeding.

There are many different types of colitis with different causes. Some examples of colitis include:

• infectious colitis caused by bacteria (such as shigella, Campylobacter, E. coli, and C. difficile)
  
  
• infectious colitis caused by a virus (such as cytomegalovirus [CMV])
  
  
• radiation colitis (such as following treatment with radiation for prostate cancer)
  
  
• ischemic colitis (such as blockage of an artery in the colon by a blood clot. If the blood clot interrupts the flow of blood to a segment of the colon, the result is inflammation of that segment and, sometimes, even death [gangrene] of the segment)
  
  
• Crohn's disease and ulcerative colitis (two related conditions that are caused by abnormalities of the body's immune system in which the body is inappropriately making antibodies and chemicals that attack the colon). Crohn's disease and ulcerative colitis are also referred to as inflammatory bowel disease (IBD).

Infectious, radiation, ischemic, ulcerative, and Crohn's colitis all have visible abnormalities of the inner lining of the colon. These abnormalities include edema (swelling of the lining), redness, bleeding from the lining with gentle rubbing (friability), and ulcers. These abnormalities can be seen during either colonoscopy (examination of the entire colon using a long flexible viewing tube) or flexible sigmoidoscopy (examination of the rectum and the sigmoid colon - the segment of the colon closest to the rectum).

Edema and inflammation of the colon's lining interferes with the absorption of water from the undigested food, and the unabsorbed water exits the rectum as diarrhea. Pus and fluid also are secreted into the colon and add to the diarrhea. The redness, bleeding from the lining with gentle rubbing (friability), and ulcerations in the lining of the colon contribute to rectal bleeding.

Source: MedicineNet.com
http://www.medicinenet.com/lymphocytic_colitis/article.htm