When sensitized tissue was first irradiated and then reexposed to antigen, the duration of the interval between irradiation and antigen addition determined the degree of inhibition of the secondary response obtained. A delay of 12 hr resulted in stronger inhibition than a delay of 6 hr, and an interval of 24 hr before reexposure to antigen caused complete suppression of antibody production to diphtheria toxoid and almost complete suppression when sheep RBC were used as the antigen.

Induction of the secondary response in rabbit lymph node tissue in vitro followed by exposure to X-irradiation, revealed that immediate exposure to irradiation after antigen produced stronger inhibition of the subsequent response than irradiation on days 2–3. Irradiation on day 6 had no detectable effect. The effectiveness of the early radiation is probably due to prevention of the proliferation of the antibody-forming cells. BUDR was found to be effective at similar time periods as X-irradiation, whereas colchicine could still stop antibody formation when added late during the secondary response in vitro.

It was noted that lymph nodes from some BSA-sensitized rabbits as late as 18 months after sensitization gave a response indistinguishable from a typical secondary response, even when not reexposed to antigen.