Microscopic colitis is an inflammation of the colon that a health care provider can see only with a microscope. Inflammation is the body’s normal response to injury, irritation, or infection of tissues. Microscopic colitis is a type of inflammatory bowel disease-the general name for diseases that cause irritation and inflammation in the intestines.

The two types of microscopic colitis are collagenous colitis and lymphocytic colitis. Health care providers often use the term microscopic colitis to describe both types because their symptoms and treatments are the same. Some scientists believe that collagenous colitis and lymphocytic colitis may be different phases of the same condition rather than separate conditions.

In both types of microscopic colitis, an increase in the number of lymphocytes, a type of white blood cell, can be seen in the epithelium-the layer of cells that lines the colon. An increase in the number of white blood cells is a sign of inflammation. The two types of colitis affect the colon tissue in slightly different ways:

__Lymphocytic colitis. The number of lymphocytes is higher, and the tissues and lining of the colon are of normal thickness.
__Collagenous colitis. The layer of collagen, a threadlike protein, underneath the epithelium builds up and becomes thicker than normal.

When looking through a microscope, the health care provider may find variations in lymphocyte numbers and collagen thickness in different parts of the colon. These variations may indicate an overlap of the two types of microscopic colitis.