Guinea pigs were injected intravenously with lymphoid cells sensitized to tubercle bacilli (TBC cells) and with lymphoid cells sensitized by contact to a simple chemical, dinitrofluorobenzene (DNFB cells). In each transfer, either the TBC cells or the DNFB cells were labeled with H3-thymidine. Immediately after transfusion, each recipient was skin tested with PPD and DNFB. 24 hours later these lesions were removed for determination of total radioactivity and for autoradiographic analysis. When TBC cells labeled with H3-thymidine were transferred with DNFB cells without an isotopic marker, the total radioactivity and the concentration per gram of skin lesion were greater in the PPD test sites. In the reciprocal arrangement, when DNFB cells labeled with H3-thymidine were transfused with TBC cells without an isotopic tag, the total radioactivity and the concentration per gram of skin lesion were greater in the DNFB test site. Similar results were obtained in guinea pigs which were actively immunized by tubercle bacilli and passively by transfer of DNFB cells. Autoradiographic analysis of test sites from guinea pigs passively transferred with both types of sensitized cells confirmed these findings.

By calculation, only a very small number of transferred sensitized cells reached the specific test lesion. Most of the cellular infiltrate was derived from the responding host. The specificity of the reaction of delayed hypersensitivity was apparently achieved by retention of the sensitized cells after they had arrived by chance at the specific antigen depot and was not due to a non-specific stickiness of sensitized or inflamed lymphoid cells.

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