Different populations of T cells have evolved to combat different types of infectious pathogens. Thus, cellmediated immunity provides excellent examples of the specialization of adaptive immunity. There are several important general principles of cell-mediated immune reactions.
• Effector T cells of the CD4+ lineage link specific recognition of microbes with the recruitment and activation of other leukocytes that destroy the microbes (Fig. 10-1A). The nature of the leukocytes that are recruited and activated is determined by the subset of CD4+ effector T cells that are induced in the immune response. In general, TH1 cells activate macrophages, Th17 reactions are dominated by neutrophils (and variable numbers of macrophages), and TH2 cells recruit and activate eosinophils. Each type of leukocyte is specially adapted to destroy particular types of microbes. This cooperation of T lymphocytes and other leukocytes illustrates an important link between adaptive and innate immunity: by means of cytokine
FIGURE 10-1 Types of T cell-mediated immune reactions. A, CD4+ T cells recognize antigens of phagocytosed and extracellular microbes and produce cytokines that activate the phagocytes to kill the microbes and stimulate inflammation. CD8+ T cells can also secrete cytokines and participate in similar reactions. B, CD8+ cytotoxic T lymphocytes (CTLs) recognize antigens of microbes residing in the cytoplasm of infected cells and kill the cells.
Phagocytes with ingested microbes in vesicles
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