Abstract
The ease with which delayed hypersensitivity can be transferred in normal man is very puzzling. Whereas large numbers (the entire lymph node or peritoneal cell population from 1 or 2 animals being required for a single transfer) of viable cells must be administered intravenously to effect a transfer of tuberculin sensitivity that lasts a week or two at most in the guinea pig, in man the leukocytes from a mere 50 to 100 ml. of blood can transfer sensitivity that lasts for a year or more. Furthermore, it is striking that in man dead cells and cell-free preparations work as . . .

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