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The intestinal epithelium is colonised by, among other cells, normal T helper and cytotoxic T cells which obtain their antigen experience in the gut associated lymphatics after migration from the thymus. New evidence suggests a CD8 positive T cell that directly migrates to the intestinal epithelium increasing the role of the thymus in shaping intestinal immunity
A normal splenic T cell would be abhorred should it get close to the intestinal epithelium. Instead of its usual rather sterile environment, it would be bathed in an ocean of usually non-pathogenic microbial and food derived antigens never seen during its passage through the educating thymus. It would get ready to become very excited. However, although the number of T cells in the mouse small intestine is approximately 50% of those residing in all lymphoid organs,1,2 there is peace. This peace is the result of a balanced presence of activated protecting T cells and activated controlling T cells.
The gut represents a unique dilemma for the immune system. It has the largest surface of the body harbouring numerous species of microorganisms. It is thus challenged with the task to be a barrier to the outside world and also to be the organ for most effective absorptive and secretory functions. To be able to accomplish this task, mammals have evolved a special highly regulated protective immune system comprising T cells which are on constant alert, as is evident from their activated phenotype. Broadly, the gut harbours three categories of T cells. Those in the gut associated lymphoid tissues are very similar to T cells found in peripheral lymphoid organs. Another population of T cells is scattered between other different cell types in the lamina propria. Their education follows the standard path of meeting their antigens presented by antigen presenting cells in the …
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