Fresh thyroids transplanted to I-region disparate recipients are, in most cases, rejected; in some instances fresh thyroids undergo periods of crisis followed by functional recovery. Cultured thyroids that are taken from donor animals pretreated with lymphocytotoxic drugs, gamma radiation and cultured for 10 d in vitro are not rejected by any normal allogenic recipient. If the recipient is sensitized with lymphoid cells syngeneic with an I-region disparate cultured thyroid donor, the cultured thyroid is rejected if I-A-subregion differences are included. We interpret these data to indicate that there exist I-region encoded perenchymal cell target determinants which are not, by themselves, immunogenic.

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