Toxic (irritant) contact dermatitis was elicited by epicutaneous application of dinitrochlorobenzene (DNCB) and croton oil in unsensitized guinea pigs. The course of the skin reaction was studied with the naked eye, low-power microscopy, and a method based on counting of infiltrating cells in the upper corium. A weakly toxic dose of DNCB (100 microgram/cm2) gave marked erythema and moderate swelling, more pronounced 6 h after application of the DNCB than at 24 h or 48 h. A significant increase in mononuclear cells in particular but also in neutrophil and eosinophil granulocytes was noted in the upper corium. A more strongly toxic dose (500 microgram/cm2) gave a similar visible reaction, this too most marked at 6 h, but here there was an increase in neutrophil granulocytes in particular. Over and above predominant mononuclear-cell infiltration, croton oil (10 microgram/cm2) caused a slight increase in basophil cells. The reaction thus resembled a contact allergic reaction. In the light of earlier findings in allergic contact reactions the results suggest that weak stimuli, toxic or allergenic, elicit a non-specific skin response. With stronger stimulation the histological picture is modified and the cellular response acquires a pattern characteristic of toxic or allergic contact dermatitis.

We describe a quantitative method for the grading of contact allergic reactions in guinea pigs. These reactions are characterized by marked cellular infiltration, and the method is based on total and differential counting of cells in the upper corium. A varying and objectively gradable increase in mononuclear and basophil polymorphonuclear cells was found. In naked-eye-positive reactions this increase was highly significant 24, 48, and 72 hours after epicutaneous application of dinitrochlorobenzene (DNCB). The degree of cellular infiltration reflects aspects of a cell-mediated immune response other than the visible reaction ordinarily made use of. The method can be used to study how systemically or topically administered drugs affect cellular features in contact allergy.