It's really not. The gut contains most of the immune system, and is most of the surface area in contact with the outside. The microbiome controls how much con

It's really not. The gut contains most of the immune system, and is most of the surface area in contact with the outside. The microbiome controls how much contact the immune system has with the gut lumen (intestinal permeability) and the chemical products it has to neutralize (like LPS). Through cytokines or movement through the lymphatic system, immune cells in the gut set the tone for the immune system systemically. When you consider how many diseases are related to a dysfunctional immune system, "most" isn't really an exaggeration. It's especially not an exaggeration if you weight diseases by how common they are.