What makes a chemical a respiratory sensitizer?

Purpose of review: In this article we consider the characteristics that are associated with chemical respiratory allergens, and that may be essential for effective sensitization of the respiratory tract. Recent findings: Chemical respiratory allergens share some characteristics with other chemical allergens, specifically chemical allergens that cause skin sensitization and allergic contact dermatitis. The unique and defining characteristic of chemical respiratory allergens, which in most instances distinguishes them from contact allergens, is the ability to provoke the preferential development of T helper 2-type immune responses. There are, in addition, other characteristics, such as the ability to increase matrix metalloproteinase expression or to cause perturbation of redox homeostasis, that may in some instances facilitate the induction or expression of respiratory allergy, but it is not yet clear if these attributes are common or essential properties of all chemical respiratory sensitizers. Summary: Predicting which chemical allergens may selectively induce respiratory sensitization is an important objective, but remains a significant challenge because our understanding of the relevant physicochemical characteristics and biological properties that confer on chemicals respiratory allergenic potential is incomplete. © 2005 Lippincott Williams & Wilkins.