Allergic Contact Dermatitis: Immunopathology and Potential Therapeutic Strategies

Allergic contact dermatitis (ACD) is a common inflammatory skin disease induced by exposure of the skin to contact allergens. Classically, ACD is defined as a delayed-type (type IV) hypersensitivity reaction mediated by allergen-specific T cells, with symptoms peaking 48-72 h after exposure to the contact allergen. This delayed response to the contact allergen is seen during patch testing, where allergen-naïve, unaffected skin of allergic individuals is exposed to the contact allergen. However, in daily life and in certain occupational settings, allergic individuals often experience rapid flare-ups/exacerbations with intensely itching erythema, oedema, and often vesicles within hours after re-exposure to the specific contact allergen. These rapid flare-ups only develop at skin sites previously exposed to the contact allergen. Thus, it is important to distinguish between the rapid-onset reaction typically experienced by the allergic individual and the delayed-type reaction typically seen after patch testing. This review summarizes current insights into the immunopathology of rapid- versus delayed-type ACD reactions and outlines potential therapeutic opportunities, as well as their current limitations, against rapid-onset ACD, including modulation of cytokine signaling, T cell survival, checkpoint pathways, and redox balance.