Chapter 18 The skin/Fig 18.05.jpg

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Contact allergic dermatitis. The immunopathogenesis of this delayed-type hypersensitivity reaction is presented diagrammatically. Langerhans’ cells process antigens (pale blue triangles) applied to the epidermal surface. During processing, the antigen is modified (dark blue triangles) which enhances its immunogenicity before being presented to T lymphocytes (green discs) (A). T lymphocytes exposed to antigen then migrate to regional lymph nodes, where there is clonal expansion of specifically sensitized cells (B). Subsequent exposure to the antigen results in chemical signalling which stimulates the proliferation of specifically sensitized T lymphocytes. Cell adhesion molecules allow these lymphocytes to home into the area of re-exposed epidermis where they elaborate the cytokines responsible for eliciting spongiotic dermatitis (C).