The hypothesis is formulated, which explains genesis of long-lasting disturbances in the immune system of the persons affected by factors of Chernobyl disaster. Immunological alterations which are displayed at the late time after action of radiation in doses 0.5 Gy or lower are not a result of direct damage of the cells of immune system by irradiation. Their development is more probably a result of appearance of some systemic conditions and factors in affected organism--such as hormonal disbalance and especially autoantibodies of different specificities, including those reactive with thymic epithelial cells. Autoantibodies of the last type induce the decrease of thymic hormone secretion which results in functional deficiency of T lymphocytes. This chain of events is similar to those occurring in aging and does not directly causes development of the clinical displays of immunodeficiency. Only irradiation in doses of 4-6 Gy or higher inducing the structural damage of thymic microenvironment can rouse the long-lasting T cell immunodeficiency with the clinical manifestations.