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Cytotoxic T lymphocytes (CTLs) are generated by immune activation of cytotoxic T cells (Tc cells). They are generally CD8+, which makes them MHC class I restricted. CTLs are able to eliminate most cells in the body since most nucleated cells express class I MHC molecules. The CTL-mediated immune system can be divided into two phases. In the first phase, functional effector CTLs are generated from naive Tc cells through activation and differentiation. In the second phase, effector CTLs destroy target cells by recognizing the antigen-MHC class I complex. == Phase 1 == In phase one, effector CTLs are generated from CTL precursors. The CTL precursors include naive Tc cells since they are incapable of killing target cells. After a precursor cell has been activated, it can then differentiate into a functional CTL with cytotoxic activity. There are three sequential signals that are required to complete this process. First, there is TCR recognition of the peptide-MHC class I complex. This step allows the cell to become licensed to an antigen presenting cell. Second, a costimulatory signal is transmitted by the interaction between CD28 and B7 of the precursor cell and the licensed antigen presenting cell. Last, a signal is induced by the interaction between IL-2 and the high-affinity IL-2 receptor. This results in proliferation and differentiation of the antigen-activated precursor cell into a functional effector CTL. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「CTL-mediated cytotoxicity」の詳細全文を読む スポンサード リンク
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