It was shown using the model of L2-dependent proliferation of T-cells that IL2 forms complexes with gamma-globulin inhibiting IL2 activity. In this interaction IL2 acts as lectin binding with two-antenna terminals of 5 or 6 mannose residues in one and the same region of immunoglobulin. Similar effect is observed in vivo in case of induction of DTH to listerial antigens. This phenomenon accounts for suppression of the immune response after passive injection of autoantibodies and for the inverse relationship between humoral and cellular immunity. Reduction of IL2 activity after its interaction with IgG or immune complexes may be regarded as a universal feedback immunoregulatory mechanism. External and internal factors showing affinity to mannose, e.g. sodium polyprenyl phosphate or lactobacillus, can interfere with this process and exert immunomodulating effect.