Your digestive tract is the primary place of interaction between your immune system and foreign cells. The 3-5 pound mass of foreign cells that exist at any one time in your gut actually contain more cells than your body! In essence, this foreign cell mass is a type of "metabolic organ" that is vital to your digestion and health. 

Much of your immune system is located in close proximity to these foreign cells, in an effort to keep tabs on what is going on and to ensure everything remains in a friendly state of balance. Many types of bacteria and the yeast Candida albicans are supposed to be in a friendly state of happy ...