Studies on the effect of systemic administration of sensitizers to guinea-pigs with contact sensitivity to inorganic metal compounds. V. Studies on the mechanism of the 'flare up' reaction.

The mechanism of the `flare up' reaction of previous epicutaneous skin test sites in guinea-pigs sensitized to potassium dichromate seems to be similar to that of the Arthus phenomenon and different from that of contact skin sensitivity. No circulating antibodies were found in the serum of these animals but it was possible to transfer the capacity of developing `flare up' reactions by an intradermal injection of peritoneal exudate cells from sensitized donors to normal recipients in which a `flare up' could be elicited by intravenously injecting the hapten 4 days later. `Flare up' reactions could also be transferred by cells from animals permanently desensitized to contact hypersensitivity to chromium. This is in agreement with the ability to produce `flare up' reactions in desensitized animals. The assumption that the `flare up' reaction is produced by humoral antibodies released by sensitized lymphocytes in the skin test site is further supported by the finding of an activity in the supernatant of cultures of sensitized lymphocytes with a chromium conjugated guinea-pig serum which when injected intradermally into normal recipients can produce an Arthus-like reaction.