Before I became a gastroenterologist, I received a PhD in immunology. I see the gastrointestinal tract as the most immunologically interesting and important organ in the body. While most of the body's barriers (skin, lungs, etc) are designed to keep out foreign materials, the gut's role is to somehow let in critical nutrients we ingest, without activating an immune response to the food we eat. At the same time, the immune system must prevent any of the myriad germs that live in our intestines from entering out blood and tissues to cause infection. Understanding how this works in health helps us understand how the immune system's discretion may break down in disease. While this is still a very active area of research, it has already produced powerful tools for the treatment of GI inflammatory diseases, which only promise to improve as we learn more.