Not only is the gastrointestinal tract the largest immune organ in the body, but it also contains one of the most important and interesting immunologic compartments. Host protection against pathogens and injurious agents by the gastrointestinal tract is essential for an individual's survival. The intestinal mucosal immune system, which is linked with other mucosal surfaces and together represents the common mucosal immune system, prevents the passage of potentially harmful antigens and pathogens into the systemic circulation of the host. In a healthy host, antigens crossing the mucosal barrier in physiologic quantities evoke the appropriate immune response, which includes polymeric IgA antibody production to the antigen and systemic tolerance.

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