TYPE IV: DELAYED
T cells drive this reaction when they react with antigen and release TH1 cytokines. The cytokines in turn attract other cells, such as macrophages, which release their lysosomal enzymes. Histologically, the lesions consist of lymphocytes, macrophages, and occasionally eosinophillic polymorphonuclear leucocytes, leading to a chronic lesion of necrosis fibrosis and granulomatosus reaction. An excellent example of this reactivity is seen when PPD is injected into the skin of a person who has been previously infected with the tuberculosis organism.
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