Irritant contact dermatitis is caused by (usually repeated) exposure to chemical or mechanical trauma. Some individuals seem more prone than others. In allergic contact dermatitis the sensitizing agent (antigen) crosses the stratum corneum to reach the epidermal Langerhans cells. The antigen is processed by the Langerhans cells and presented to circulating T lymphocytes with subsequent development of a clone of T lymphocytes with a specific memory for that particular antigen. In an individual sensitized to a particular antigen, repeat exposure to that antigen will result in migration of the sensitized T lymphocytes to the site of exposure, with initiation of the inflammatory eczematous process. Both irritant and allergic contact dermatitis usually start at the site(s) of initial or more frequent contact, but may spread to involve other areas.
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