Delayed hypersensitivity or Type IV reactions (Table 11) are T-cell-mediated, i.e., specifically sensitized CD4+ T-lymphocytes initiate the reactions. Sensitization develops slowly requiring repeated exposures to a specific allergen. Once sensitized, upon reexposure immunologically committed lymphocytes react with the allergen (antigen) and damage tissue by direct toxic effects or through the release of cytokines, which activate eosinophils, monocytes, neutrophils, and macrophages and killer cells.
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