CTL-mediated killing involves specific recognition of target cells and delivery of proteins that induce cell death.
CTLs kill targets that express the same class I-associated antigen that triggered the proliferation and differentiation of naive CD8+ T cells from which they are derived and do not kill adjacent uninfected cells that do not express this antigen. In fact, even the CTLs themselves are not injured during the killing of antigen-expressing targets. This specificity of CTL effector function ensures that normal cells are not killed by CTLs reacting against infected cells. The killing is highly specific because an "immunologic synapse" (see Chapter 6) is formed at the site of contact of the CTL and the antigen-expressing target, and the molecules that actually perform the killing are secreted into the synapse and cannot diffuse to other nearby cells.
The process of CTL-mediated killing of targets consists of antigen recognition, activation of the CTLs, delivery of the "lethal hit" that kills the target cells, and release of the CTLs (Fig. 10-11). Each of these steps is controlled by specific molecular interactions.
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