Fig. 1.1
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)
1.2 Allergens
Most of the contact allergens are small and chemically reactive molecules not exceeding a molecular weight of 800 Dalton. Due to their size, they can penetrate through the epidermal barrier. In the epidermis and dermis, they can react with endogenous peptides and form immunologically relevant allergen-carrier complex. For some allergens, an enzymatic of metabolic activation step is needed to generate the actual allergen.
In principal, all contact allergens have to a certain extent irritant properties. This irritancy can add to the allergenic potency by triggering the release of innate danger signals from immune cells.
1.3 Antigen-Presenting Cells
In the epidermis and dermis are different types of professional antigen-presenting dendritic cells. Their common feature is the capacity to pick up and present antigens to other cells of the immune system. The epidermal antigen-presenting cells are called Langerhans cells, and the dermal types are summarized as dermal dendritic cells. Upon contact with contact allergens, dendritic cells get activated. The release of pro-inflammatory danger signals from surrounding cells or directly from the dendritic cells amplifies this activation. Under the inflammatory pressure, dendritic cells get fully matured and start to emigrate from (epi)dermal structures via the lymphatic vessels toward the draining lymph nodes. Here, they get attracted by chemokines binding to the chemokine receptor CCR7, which is expressed on the cellular surface of matured dendritic cells. This allows antigen-presenting cells to settle in the subcapsular compartments of the draining lymph nodes.
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