T lymphocytes injure tissues either by triggering inflammation or by directly killing target cells (Fig. 18-5). Inflammatory reactions are elicited mainly by CD4+ T cells of the TH1 and TH17 subsets, which secrete cytokines that recruit leukocytes. In some T cell-mediated disorders, CD8+ CTLs kill target cells bearing class I major histocompatibility complex (MHC)-associated antigens. The T cells that cause tissue injury may be autoreactive, or they may be specific for foreign protein antigens that are present in or bound to cells or tissues. T lymphocyte-mediated tissue injury may also accompany strong protective immune responses against persistent microbes, especially intracellular microbes that resist eradication by phagocytes and antibodies.
A role for T cells in causing a particular immunologic disease is suspected largely on the basis of the demonstration of T cells in lesions and the isolation of T cells specific for self or microbial antigens from the tissues or blood of patients. Animal models have been very useful for elucidating the pathogenesis of these disorders.
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