Delayed hypersensitive guinea pigs can be partially desensitized in a stepwise manner with increasing doses of antigen administered parenterally prior to skin testing. There is a close correlation between the size of the desensitizing dose of antigen and the increase in the amount of antigen required at the skin test site to elicit a response, suggesting the existence of a heterogeneity of binding energies for antigen in the delayed system. Data are presented showing that delayed

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... y responses can be elicited in partially desensitized guinea pigs with residual circulating antigen concentrations as high as 8 × 10-8 M, while undesensitized animals will respond to as little as about 10-14 moles of antigen. These data are interpreted to argue against the participation of low concentrations of highaffinity humoral antibodies in the development of delayed hypersensitivity lesions.