Rats subjected to high doses of whole-body irradiation, with simultaneous shielding of the thymus or spleen, recovered at 3 wk the ability to develop delayed sensitization and to form hemagglutinating and precipitating antibody following foot-pad injection of BγG in complete adjuvant. Injection of BγG into the shielded thymus immediately after irradiation, in amounts between 20 γg and 40 mg, inhibited these response to later challenge partially or completely. A comparable effect on immune responses to BγG was not seen after injection of heterologous antigen (Ea) intrathymically, BγG intraperitoneally, or BγG into the shielded spleen. However high doses (20 or 40 mg) of antigen given by the latter routes resulted in some diminution of later response. Arthus reactivity recovered partially in the spleen-shielded group and was readily suppressed by intrasplenic administration of antigen.

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