Beside the variety of environmental and neurohormonal factors regulating the gastrointestinal function, microbes and parasites can also modulate this function. Their influences can be harmful, thus the gastrointestinal tract is equipped with numerous specific or unspecific defense mechanisms. While the gastric acid secretion in the stomach, gastrointestinal mucosal continuity, or some adaptation mechanisms are unspecific ones, the immunological gastrointestinal system comprises a cluster of specific defense mechanisms. The identification, binding and neutralization of the aforementioned antigens serves as its principal role. The immunological system is well developed in the gastrointestinal tract and might be understood as the main barrier protecting the organism against the infectious factors. The gastrointestinal immunological system is localized mainly in subepithelial space, where the lymphocytes and a substantial amount of immunoglobulin A are present. Secretory immunoglobulin A also occurs in the intestinal lumen and is able to block the adhesion of antigen proteins to intestinal epithelium. However, the portion of antigens passing the gut epithelium migrating into the subepithelial space where it is neutralized, at least in part, can spread over the body in at least three different pathways: to Peyer patches and then toward the mesenterical lymph nodes and finally to a lymph, directly to the lymphatic vessels of intestinal villus or to the mesenteric veins. In addition, the antigens evoke various immunological reactions. Gastrointestinal immunological processes can be amplified (sensitization) or hampered (tolerance). The nervous system can also participate in immunological reactions linked with the gastrointestinal tract. Thus, in the gastrointestinal tract and also in the nervous system exist clear relationships between their basic functions and immunological processes.
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