Allergic contact dermatitis is an inflammatory reaction that follows absorption of antigen applied to the skin and recruitment of previously sensitized, antigen-specific T lymphocytes into the skin. It affects a limited number of individuals. The antigens are usually low-molecular-weight substances that readily penetrate the stratum corneum. Most contact allergens are weak and require repeated exposure before sensitization occurs. Strong antigens, such as poison ivy, require only two exposures for sensitization.
Interaction between antigen and T lymphocytes is mediated by antigen-presenting epidermal cells (Langerhans' cells) and is divided into two sequential phases: an initial sensitization phase and an elicitation phase. Langerhan's cells are abundant in skin and sparse at mucosal sites
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