Methods were described in the previous paper for contact sensitization and tolerization to DNCB and to PCI, and for the detection of specifically reactive cells in vitro. These methods have now been used to detect regulating serum factors. The reactivity of PC from tolerant mice in LAI tests in vitro was blocked by serum from tolerant mice. Hapten specificity of blocking was demonstrated. Serum from hypersensitive mice lacked these blocking factors, and cells from such mice could not be blocked in vitro. Hypersensitive serum was able to counteract (unblock) the activity of tolerant serum. Both of these activities were absent in serum from cyclophosphamide-pretreated animals, and cells from such animals could be blocked in vitro. Provided recipients were properly prepared (by prior treatment with cyclophosphamide), hapten-specific tolerance to contact-sensitizing agents could be transferred by tolerant serum. In addition, tolerance could be terminated abruptly by suitably sensitized serum. These two activities corresponded to factors demonstrable in vitro. A further suppressive property of sensitized serum, namely the ability to inhibit partially the enhanced cutaneous reactions of cyclophosphamide-treated mice, was not apparent in vitro. The possible nature and origins of these serum factors are discussed.
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