The gastrointestinal tract (GIT) performs not only digestive but also immune function, in particular, participates in the implementation of protective reactions of the body against pathogenic, conditionally pathogenic microorganisms and many inorganic substances. Therefore, it is essential to keep the intestines healthy, and the body’s immune system will be much stronger in this case.
The epithelium of the gastrointestinal tract delimits the tissues of the macroorganism from a huge number of living antigens and nonliving antigens, substances that carry signs of foreign genetic information. Oral exposure to an antigen (including microbes and their toxins) usually creates, on the one hand, local mucous IgA protection (secretory immunity) and a cell-mediated reaction, but, on the other hand, systemic tolerance or hyporeactivity, the suppression of subsequent production of antigen-specific antibodies of classes G and M and the development of cell-mediated immune system. Concerning pathogenic and conditionally pathogenic microorganisms, the local immune system of the intestine must show adequate protective properties, and concerning normal flora, at least tolerance, and at best, actively participate in the processes of adhesion, survival and reproduction of representatives of normal flora. The intestines and the body’s immune system are tightly interconnected.
Specific immune mechanisms are produced by the intestines to protect against potentially dangerous microorganisms throughout human life. Undifferentiated lymphocytes, which produce mostly secretory IgA or IgM antibodies, are present in the mucosal layer or Peyer’s patches. Stimulation of B-and T-lymphocytes in the presence of a foreign antigen occurs after their release from the mesenteric nodes into the thoracic duct, bloodstream and return to the intestine, where they also accumulate in the mucosal layer. Activated cells produce specific antibodies of classes IgA and IgM, which are secreted on the surface of the mucous membrane 48 days after stimulation. Immunoglobulins form complexes with antigens, neutralise toxins, prevent contact of microorganisms with target cells of the macroorganism, and promote the rapid removal of microorganisms from the gastrointestinal tract due to agglutination.
The primary function of intestinal antibodies is immune rejection at the mucosal surface. Such a relationship as the intestine with the body’s immune system is vital for humans.
The most important property of the local intestinal immune system is the phenomenon of lymphocyte recycling. Peyer’s patches lymphocytes, sensitised by antigens, both food and infectious, migrate to the mesenteric lymph nodes, and from there through the lymphatic vessels through the thoracic duct and circulatory system are sent to their own layer of the intestinal mucosa, mainly as IgA secreting cells. This mechanism ensures the formation of lymphocyte clones and the formation of specific antibodies in the mucous membrane areas remote from the source of primary sensitisation. Native immunoglobulin molecules, as well as active Fc and F (ab ) 2 fragments, participate in the sensitisation of plasma cells with the subsequent cloning of lymphocytes that produce antibodies with specific properties similar to those that played the role of a matrix.
Cellular immunity of the intestine, in contrast to the system of antibodies secreted by it, has not been studied enough. It is known that systemic cellular immune responses are rarely detected after oral exposure to antigens. It is evident that cellular immunity reactions do not develop in the intestinal mucosa, when healthy people receive harmless antigens, for example, normal flora antigens.