The article is designed as a lecture for students of medical universities, as well as for practicing doctors. It shows the periods of childhood with characteristic for each stage features of a growing organism.

The indices of T-cell immunity are decreased in children of early age with megalothymus: content of T-cells (CD3+), Т-helpers (СD4+ Т-cells), cytotoxic Т-lymphocytes (СD8+ Т-cells), regulatory Т-cells (CD4+CD25hi) and also activated Т-lymphocytes (CD4+CD25lo и CD3+HLA-DR+). The increasing of expression of alterations was observed as degree of megalothymus was progressing. The only exclusion were regulatory T-cells. Their level was decreased under megalothymus degree I and II and was closer to indices of age standard under megalothymus degree III. Also, it was demonstrated that capacity of T-cells to differentiation to cytokine-producing cells type Th1 and Th2 increases as far as increases expression of megalothymus. However, this process remains within framework inherent to comparison group. The T-lymphopenia under megalothymus is conjugated with decreasing of emigration of T-cells from thymus to peripheral section of immune system. These alterations bring on deficiency of T-cell chain of immune system and can favor manifestation of its incapacity especially in conditions of increased load under effect of pathogens.

The article demonstrates that in children with thymomegalia the process of T-lymphopoesis in thymus is depressed. The functional condition of thymus was evaluated by detection of T-receptor excisional rings using polymerase chain reaction technique in real time. The T-receptor excisional rings are circular molecular structures developing in the process of rearrangement of V-genes of T-cell receptor. The most intensive and statistically reliable decreasing of content of T-receptor excisional rings in lymphocytes of blood is detected during first two years of life: 29.6 (18.1-41.0) copies per 1000 lymphocytes against 73.3 (54.0-83.8) copies in control. Hereinafter, (after two years) difference in level of T-receptor excisional rings inc children with thymomegalia in comparison with children without enlargement of thymus is less expressed i.e. it amounts to 1.8 times at the age before 10 years. In individuals of older age thymomegalia is a rather rare pathology. The examination of single patient aged of 40 years and with thymomegalia factually total absence of T-receptor excisional rings in cells of his blood was established.