Role of the Thymus in Recovery of the Immune Mechanism in the Irradiated Adult Mouse

Summary 1. Inbred mice were thymectomized at 2 months of age, given lethal doses of irradiation and syngeneic marrow therapy. Controls were either sham-thymectomized and irradiated or sham-irradiated and thymectomized. 2. Thymectomized irradiated mice failed to reject allogeneic skin grafts and to produce either a primary or a secondary immune response to sheep erythrocytes when tested 4–10 weeks after irradiation. At that time, control mice had normal immune functions. 3. Lymphocytes were markedly diminished in the experimental and the thymectomized control groups. They were present and within normal limits in sham-thymectomized irradiated controls. The depression in the thymectomized irradiated mice characteristically involved the small mononuclear cells. 4. It is concluded that recovery of the immune mechanism following total body irradiation takes place by means of a thymus-dependent mechanism.