The specificities of arrangement and normal function of the intestinal immunological system are presented, and non-immunological and immunological gastrointestinal defense mechanisms are described. Of immunological defense mechanisms, cellular and humoral mechanisms are described separately, and their characteristics in childhood are highlighted. After a general survey of the intestinal immunological system disorders and their role in various diseases, three most frequent such diseases are described in detail: food intolerance, gluten enteropathy and chronic inflammatory bowel disease.