As I’ve mentioned earlier, the colon is where the majority of bacteria in your gastrointestinal tract reside, giving the cells that line it two important functions: serving as a blockade against pathogens and bacteria that don’t belong in your circulation, while also allowing for the absorption of fluids and any remaining nutrients that haven’t been absorbed by the small intestine. This physical barrier forms part of the body’s innate immune system.
The innate immune system plays a major role in mediating inflammation and autoimmunity. It helps keep the microbiome and our immune cells (our adaptive immune system) separate from each other, regulating host-microbial interactions and maintaining appropriate immune function on a constant basis. In our stadium metaphor, this allows the game to go on as planned, ensuring a nice day for everyone there.
he guards can safely perform their jobs, the fans can eat their hot dogs and cheer for their respective teams, and the players can compete, allowing them to earn millions of dollars in endorsement money. Physical barriers help make all this possible. The cells of the epithelium—your intestinal lining—are held together by tight junctions that can open and close like the drawbridge on a castle. Thankfully, they are closed most of the time.
However, exposure to potentially dangerous bacteria, especially in the small intestine, can cause the junctions to loosen, drawing water and immune cells into the gut lumen. This usually results in diarrhea to flush out the troublemaker—a critical defense response during acute infection. 20 Unfortunately, certain aspects of modern life can also cause our gut barrier to be more porous and allow fo r retrograde transport, or the transport of gut contents deep into the gut lining. This leads to considerable consequences and possibly initiates the “molecular mimicry” that is thought to result in autoimmunity