Passive transfer of tritiated thymidine-labeled BCG sensitized lymphoid cells into homologous guinea pigs resulted in positive tuberculin skin reactions 24 hours after testing with PPD. Labeled cells were found specifically attracted to these sites. Labeled non-sensitized lymphoid cells did not appear at PPD injection sites, nor did labeled sensitized cells accumulate in non-specific inflammatory lesions. The specifically reacting tritiated cells were small, medium, and large lymphocytes and stem or immature cells of the lymphoid series. In the homologous system employed, positive skin tests were either minimal or absent 3 days after transfusion of activated cells. The injected labeled sensitized cells were rapidly cleared from the circulating blood and lodged in the lungs, spleen, and lymph nodes. Upon application of specific antigen they reappeared at the skin test site at 6 hours and then increased in number for the next 18 hours.

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