Major immunological events in allergic contact dermatitis. In the induction phase of allergic contact dermatitis (left side of the drawing), skin contact with a contact allergen triggers migration and maturation of antigen-presenting cells (APC). These cells reach via the afferent lymphatic vessels the regional skin-draining lymph node. Allergen-presenting dendritic cells home into the T-cell-rich paracortical areas. Here, local conditions are optimal for encountering naive T cells that recognize allergen–MHC molecule complexes. During T-cell priming, hapten-specific T cells strongly proliferate and generate effector and memory cells, which are partly released into the circulation. Renewed allergen contact leads to the elicitation reaction (shown at the right side). Allergen-specific effector T cells are triggered to produce pro-inflammatory cytokines. Thereby, more inflammatory cells are recruited to the allergen contact site which results in strong local inflammatory mediator release. This leads to a gradually increasing inflammatory reaction, reaching a maximum within one to few days, after which reactivity successively declines (Kanerva’s Occupational Dermatology, 2012 Editors: Thomas Rustemeyer, Peter Elsner, Swen-Malte John, Howard I. Maibach et al. ©Springer-Verlag Berlin Heidelberg 2012, with Permission of Springer Science+Business Media)