Thresholds in contact sensitization: Immunologic mechanisms and experimental evidence in humans—an overview

The evidence from practical experience in man, from controlled human testing and from both chemical and biological mechanistic considerations, demonstrates that the thresholds for skin sensitization exist both at the level of induction as well as sensitization. For a given contact allergen, the concentration (expressed in dose per unit area), which is at the threshold, has to be defined in terms of the nature and extent of the skin contact involved, and whether it is the induction of skin sensitization or its elicitation that is involved, since the numerical values will depend heavily on these parameters. The induction and elicitation of contact allergy is influenced by several factors: the total dose of the allergen, its surface concentration, the size of area over which it is applied, antigenic potency of the substance, number of exposures, anatomical site of exposure, effect of draining lymph nodes, matrix (vehicle) of allergen, effect of occlusion, degree of percutaneous penetration, effect of diseased skin/trauma and possibly a host of other, as yet unknown, variables.