Frequently ill children (FIC) show persistence of infection in the nasopharynx, disbiosis of intestinal flora, and concomitant and allergic diseases. As per our results, FIC with acute respiratory diseases (ARD) frequency of 6-15 times a year plus chronic infection foci at the age of 2-15 y/o at the remission period have heterogeneous nature of immune system disorders. It depends on the age, frequency of ARD, and chronic infection foci. About 20-50% of children have low number of T cells and 70% of children have high number of activated T cells. About 5-23% of children have low level of serum IgG or IgA, while low level of saliva IgA has been determined in 94% of children and low synthesis of IFN-alpha in 80% and of IFN-gamma in 30% of children. About 50% of kids have high level of common IgE (160-220 ME/ml) and diagnostic sensitization to various allergens. In contrast, only 25% of children with ARD frequency of 4-6 times a year without chronic infection foci had low synthesis of IFN-alpha, 30% had low IgA level in saliva, and 8.3% had low IgA level in serum. After vaccination against hepatitis B, antibody level to HBs-Ag and time of their circulation at FIC had been lower than in children with ARD frequency of 4-6 times a year. Examination of FIC at the remission period showed polymorphism of natural and adaptive immunity disorders associated with the immune system developmental delay and subsequent forming of chronic infection foci being an aggravating factor for these disorders.