Delayed hypersensitivity was produced in donor Lewis rats by sensitization with soluble protein antigens emulsified in complete Freund's adjuvant. Cells of their thoracic duct lymph were collected for varying periods of time and transferred intravenously to isogenic Lewis recipients. With this model the following conclusions were reached:

1. Delayed hypersensitivity was transferred by thoracic duct cells.

2. The longer the drainage of the thoracic duct, the fewer cells were needed to achieve a successful transfer. With continuing drainage the proportion of small lymphocytes decreased and large cells increased. There was, therefore, a better correlation between successful transfer of delayed hypersensitivity and the number of large cells transfused than between positive skin reactions and transfer of small lymphocytes.

3. Prolonged fistula of the thoracic duct did not diminish the skin reaction of sensitized donors to specific antigen.

4. Delayed hypersensitivity was elicited in recipients 3 wk after transfer of sensitized cells. There was evidence that delayed hypersensitivity was enhanced in recipients, possibly because of prior skin testing.

5. Total body X-irradiation abolished the lesions of passively transferred delayed hypersensitivity. Recovery of positive skin tests was observed 19 to 20 days later.

6. The lesions of delayed hypersensitivity were probably mediated by cells. There was no evidence that a circulating high affinity antibody played a role in this type of immunologic reaction.

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