Studies of the time course of the immune status characteristics after praziquantel treatment and of the nature of its side-effects on various groups of opisthorchiasis patients in the focus established significant differences in their immune response to specific treatment. The lack of alterations in the T- and B-immunity systems in aboriginal population of the focus may suggest its tolerance to helminthic antigens. The trend towards cellular immunity stimulation and a significant increase in the levels of circulating immune complexes and specific antibodies in the serum were observed in aboriginal population of the focus. The comers showed significant initial alterations of the immunity status before treatment, pointing to their sensitization to parasitic antigens. Suppression of the infection normalizes several cellular and humoral immunity characteristics--the number of T-helpers, T-lymphocytes, B-lymphocytes, Th/Tc indices, T-lymphocyte functional activity, thus eliminating immune depression. The increased IgG, CIC and specific antibody levels significantly exceeding the normal level, occurrence or enhancement of allergic reactions point to the necessity of performing a desensitizing therapy and using effective cholagogic drugs in the multimodality treatment to rapidly eliminate the parasitic antigen.