The present study offers direct evidence that cells from the regional lymph node draining first-set skin homografts in rabbits participate in the lymphoid infiltrate at distant sites of graft rejection. With the use of a newly devised method of injection, adequate amounts of tritiated thymidine could be confined to the reactive regional node, other dividing cells of the host remaining unlabeled. It was thus possible to detect the appearance of this uniquely labeled population of cells at remote test sites on the same animal, obviating the necessity for cell transfer. Under these conditions differential counts of the lymphoid elements failed to show a preponderance of labeled cells in the infiltrate accompanying homologous, compared with unrelated control, graft rejection. These results, together with other recently reported data, suggest that the specificity of the homograft reaction is probably dictated by some humoral factor that acts in concert with recently proliferated lymphoid cells to accomplish graft rejection.

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