T cell-mediated delayed hypersensitivity (DH) to human γ-globulin (HGG) can be induced in chickens by subcutaneous injection of the antigen in complete Freund's adjuvant (CFA). In the present work, it has been demonstrated that specific tolerance of the cells mediating this DH can readily be induced in both normal and bursectomized (BX) FP strain chickens by simple i.v. injection of soluble antigen, regardless of the presence of antibody production to the tolerogen. A significant degree of tolerance at the DH and helper T cell levels could be generated in BX birds by injection of as little as 0.5 mg of HGG; such a dose only induced tolerance in normal birds when it had been previously deaggregated by ultracentrifugation. Regular, nondeaggregated antigen could produce tolerance in normal animals, but only at doses of >5 mg. The tolerizing injection induced a primary antibody response in normal birds in all cases, but a secondary response could not be obtained in animals rendered tolerant at the T cell level.
Establishment of tolerance appeared to be very rapid, and animals remained refractory to induction of DH for at least 3 weeks after the tolerizing injection. The mode in which the antigen was presented to the animals appeared to be crucial in determining whether tolerance or sensitivity would be established.