Immunological Background of Allergic Contact Dermatitis

Abstract

Contact allergy has been one of the classical examples of acquired T-cell-mediated immunity since the discovery of lymphocyte-mediated reactions. Since then, crucial steps in the immunological cascade of contact allergy have been unraveled. Firstly, contact allergens have unique properties allowing for binding to endogenous peptide carriers. This binding is required to become fully immune competent. Some contact allergens even need enzymatic or chemical activation before they can form allergen-carrier complexes. Antigen-presenting cells can take up allergens directly or in a complexed form. This results in the activation of antigen-presenting cells. The activation and maturation of antigen-presenting cells are further increased by the generation of unspecific “danger signals” by the irritant capacities of contact allergens. This activation of innate immune reactions contributes to the allergenic properties of contact allergens and the activation capacities for priming of T cells. The fully matured antigen-presenting cells migrate to the draining lymph nodes where they can come into contact with naive T cells. Under appropriate conditions, allergen-specific T cells can be triggered to proliferate. Whereas in mice, CD8⁺ T cells are the principal effector cell types, and they seem to be less important in man. Here, allergen-specific T cells are mainly CD4⁺ cells. Allergen-specific T cells are characterized by the expression of skin-homing molecules and skin-specific migration receptors. They can belong to different functional types, and among them are Th1, Th2, Th17, and Th22, the best studied effector cell types. Based on these findings, different clinical pictures of contact allergy can now be better understood. Contact allergic reactions can show different clinical pictures. Not only the classical eczematous reactions but also nonclassical presentations as lichenoid and granulomatous reactions have been diagnosed as contact allergic reactions.