When one or two drops of a dilute, non-irritating solution of 2,4-dinitrochlorobenzene (DNCB) is applied to a small area of skin of the intact guinea pig, about 20 per cent of the applied material, or some derivative of it, is soon excreted in urine. In normal, as well as in specifically sensitized guinea pigs, DNCB at the site of local application becomes rapidly bound to skin protein through primary chemical bonds. Twenty-four hours after application roughly half of the material present at the local skin site is still extractable with organic solvents. Of the non-extractable dinitrophenyl groups, about 99 per cent are in epidermis, and about 85 per cent are substituted in ϵ-NH2 groups of lysine residues. Only traces of bound dinitrophenyl groups were observed in the corium. It is uncertain whether these are formed in situ, or are experimental contaminants, or are migratory epidermally formed conjugates. Even when DNCB is injected intradermally it combines predominantly with overlying epidermis and with epidermal components of hair follicles, but only slightly with corium.

The 2,4-dinitrophenyl conjugates which are localized in the deeper, viable half of the epidermis, close to the epidermal-dermal junction, are inferred to be the agents responsible for specifically evoking the allergic response in sensitized animals. Conjugates which are situated in the outer, cornified half of the epidermis are shown to be incapable of eliciting the allergic response. The results furnish a basis for interpreting a common pattern of lesions in allergic contact dermatitis as it occurs spontaneously in man.

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