Inflammatory bowel disease

Inflammatory bowel disease is not a single bout of inflammation but episodes that keep coming back. The trigger is not an invading germ: it is the body's own immune response, which reacts abnormally to the gut flora, the bacteria that normally live in the intestine and should not bother anyone.

Between those bacteria and the rest of the body there is a single barrier: the intestinal epithelium, the layer of cells that lines the inside of the intestine. Those cells are held to one another by tight junctions, which seal the spaces between them so that nothing slips into the blood. In this disease the junctions fail. The bowel wall is a wall whose joints are sealed; if they come open, what was meant to stay outside gets in. And then a vicious cycle starts: more microbes get through, the inflammation grows, and the inflammation damages the epithelium further.

Two diseases with different patterns live under the same name. Ulcerative colitis usually inflames the mucosa of the colon, the surface layer of the large intestine, starting at the rectum and moving up without a break. Crohn disease does the opposite: it leaves skip lesions, diseased stretches with healthy stretches in between, and instead of staying on the surface it goes through every layer of the wall.