 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 millionths 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. They serve two functions. They condition and repair the thin barrier, and provide a frontline defense against intestinal pathogens. These critical cells are covered with receptors called AH receptors. The AH receptor is like a lock, and for decades, researchers have been searching for a natural key to fit into that lock, to activate those receptors and sustain our immunity. And we just discovered the key is broccoli. The cruciferous vegetables, broccoli, brussel sprouts, kale, cauliflower, cabbage contain a phytonutrient that is transformed by our stomach acid into the key that fits into the AH receptor locks on our intraepithelial lymphocytes, leading to their activation. Here's a less busy diagram that illustrates the same thing, broccoli, leading to the activation of our immune foot soldiers. So now we know specific dietary compounds present in cruciferous vegetables act through the AH receptors to promote intestinal immune function. From childhood, we learned that vegetables are good for us, and most of us eat our veggies without giving much thought to the evidence behind this accepted wisdom, or to the mechanisms underlying the purported health boosting properties of a vegetable-rich diet. But now we know that specific dietary compounds found at high levels in cruciferous vegetables, such as broccoli, cauliflower, cabbage, are essential for sustaining intestinal immune function. Green vegetables are in fact required to maintain a large population of those protective intraepithelial lymphocytes. Maybe that's why vegetable intake is associated with lower risk of inflammatory bowel diseases, such as ulcerative colitis, whereas the more meat-centered Western diet is associated with higher risk of inflammatory bowel diseases. This may be because the activating receptors on our intestinal immune cells are basically a sensor for plant-derived phytochemicals. This raises a broader question. Why did our immune system evolve this requirement for broccoli and other plant foods? Well, think about it. When do we need to boost our intestinal defenses the most? When we eat. Thus, linking heightened intestinal immune activation to food intake could serve to bolster immunity precisely when it is needed. At the same time, this would allow energy to be conserved in times of food scarcity, since it takes so much energy. I mean, why remain at red alert 24-7 when you only eat a couple times a day? Since we evolved for millions of years eating mostly weeds, wild plants, dark green leafy vegetables, or as they were known back then, just leaves, by equating veggies with food, our bodies may be using them as a signal to upkeep our immune system. Thus the old recommendation, eat your veggies, has a strong molecular basis. This discovery has been all exciting for the drug companies who are looking into AH receptor-active pharmaceuticals. However, as one research team at Cambridge concluded, rather than developing additional anti-inflammatory drugs, changing diets, which are currently highly processed and low on vegetable content, may be a more cost-effective way towards health and well-being.