Immunology of allergic contact dermatitis.

Allergic contact dermatitis (ACD) is a T-cell mediated skin inflammation caused by repeated skin exposure to contact allergens. This review summarizes current knowledge on the immunology of ACD. Different phases in ACD are distinguished, i.e. sensitization, elicitation and resolution phases. We discuss contact allergen presentation and the central role of antigen presenting cells during sensitization phase. There is an extremely complex interaction of different kinds of immune cells, such as antigen presenting cells, T, B, NK lymphocytes, keratinocytes (KCs), endothelium, mast cells (MCs) and platelets, and this complex interaction is guided through orchestration of numerous cytokines and chemokines. The role of adaptive immunity has been recognized in contact hypersensitivity but we also discuss the important role of some parts of innate immunity such as natural killer T lymphocytes (NKT) and complement system. Cooperation of innate and adaptive immunity, in this case NK cells and B cells, initiates elicitation phase by complement cascade activation, vasoactive substance release and endothelial activation. KCs are not only innocent bystanders, on the contrary, they are involved in all phases of ACD, from the early phase of initiation through sending "danger" signals and activation of innate immunity, through their role in Langerhans cells (LCs) migration, T-cell trafficking, through the height of the inflammatory phase with direct interactions with epidermotropic T-cells, and finally through the resolution phase with the production of anti-inflammatory cytokines and tolerogenic presentation to effector T-cells. Th-1 and Th-17 cells are the main effector cells responsible for tissue damage. At the end, we point out several subsets of T regulatory cells, which exert down-regulatory function and regulate the magnitude and duration of inflammatory reaction.