On the other hand, many pathogens enter the body through the digestive system. The mucosal immune system must recognize dangerous microbial antigens and mount an immune reponse quickly. In the large intestine, the situation is even more complicated. One theory of oral tolerance proposes that microorganisms that bind to or invade mucosal epithelium cause the release of cytokines that prime immune system cells for generating an immune response. In the absence of these cytokines, the normal response would be tolerance.
Tolerance is essential for preventing food allergies.
Interestingly, we are tolerant to most food antigens when they are ingested. If injected into the bloodstream, most food antigens will stimulate an immune response. The mucosal immune system operates somewhat independently from the peripheral immune system and produces different kinds of antibodies and T cells. For more details, see Art Anderson's Immunology Web site .