Contact and respiratory sensitization by chemical allergens: Uneasy relationships*1

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Abstract
A variety of chemicals are able to cause allergic disease in susceptible individuals. Chemical-induced allergy may take several forms, chief among them being skin sensitization or allergic contact dermatitis and respiratory hypersensitivity. It has been found that contact and respiratory chemical allergens induce in mice qualitatively divergent immune responses consistent with the preferential activation of discrete Thelper (Th) cell subpopulations, Th1 and Th2 cells, respectively. Despite the selective stimulation of Th2type responses by chemical respiratory allergens, such materials are nevertheless able to induce contact sensitization in mice, which is a form of delayed-type hypersensitivity usually considered to be effected by Th1 cells. Indeed, the available evidence indicates that chemicals known to cause respiratory allergy in humans test positive in both mouse and guinea pig assays used for the prospective identification of skinsensitizing chemicals. The activity of chemical respiratory sensitizers in such tests has important-implications for the toxicological evaluation and classification of potential chemical allergens. Paradoxically, however, although chemical respiratory allergens show a potential for contact sensitization in predictive tests, this hazard rarely translates into an important risk of allergic contact dermatitis in humans.

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