Unlike naive T cells, effector T cells express certain effector molecules, which may be membrane bound or soluble (Table 14-2). The membrane-bound molecules belong to the tumor necrosis factor (TNF) family of membrane proteins and include the Fas ligand (FASL) on CD8+CTLs, TNF-p on TH1 cells, and the CD40 ligand on TH2 cells. Each of the effector T-cell populations also secretes distinct panels of soluble effector molecules. CTLs secrete cytotoxins (perforins and granzymes) as well as two cytokines, IFN-7 and TNF-p. As described in Chapter 12, the TH1 and TH2 subsets secrete largely nonoverlapping sets of cytokines.
Each of these membrane-bound and secreted molecules plays an important role in various T-cell effector functions. The Fas ligand, perforins, and granzymes, for example, mediate target-cell destruction by the CTL; membrane-bound TNF-p and soluble IFN-7 and GM-CSF promote macrophage activation by the TH1 cell; and the membrane-bound CD40 ligand and soluble IL-4, IL-5, and IL-6 all play a role in B-cell activation by the TH2 cell.
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