The gastrointestinal tract has a number of unique features. Its extensive surface is formed by a single layer of rapidly renewing cells, the intestinal epithelial cells. These cells are in contact with a number of other cell populations, including the largest part of the immune system, and with an excessive luminal antigen load, including vast numbers of bacteria. Furthermore two more organs, namely liver and pancreas, are part of the system. The rapid renewal of the epithelial layer, the interactions of different cell types, and the balance between cell proliferation and death, have been...