"Our greatest exposure to the environment, our body's greatest interface with the outside world, is not through our skin, but through the lining of our gut, which covers thousands of square feet. And in our intestine, all that separates us from the outside world is a single layer of cells, 50 millionth of a meter thick. The distance between the outside world and our bloodstream is less than the thickness of a sheet of paper.

In contrast, here's a layer of skin. Look at that — dozens of layers of protective cells to keep the outside world, outside of our bodies. Why don't we have multiple layers in our gut wall? Because we need to absorb stuff from food into our body. It's a good idea for our skin to be waterproof, so we don't start leaking, but the lining of our gut has to allow for the absorption of fluids and nutrients.

With such a thin fragile layer between our sterile core and outer chaos, we better have quite a defense system in place. And, indeed, that's where intraepithelial lymphocytes come in."