Chapter 21 - Section 21.4 - Study Guide - Assess Your Learning Outcomes - Page 845: 6

Activated T cells, specifically cytotoxic T cells (CD8+ T cells), play a crucial role in destroying target cells that are infected with intracellular pathogens, including viruses, or are otherwise abnormal. The destruction of target cells involves a combination of mechanisms aimed at inducing apoptosis (programmed cell death). Here's an overview of how activated T cells destroy target cells and the roles of key molecules such as interferons, perforin, granzymes, and tumor necrosis factor (TNF): **1. Recognition and Engagement:** Activated cytotoxic T cells recognize infected or abnormal target cells by interacting with antigens presented on the target cell surface via major histocompatibility complex class I (MHC-I) molecules. The interaction between the T-cell receptor (TCR) on the cytotoxic T cell and the antigen-MHC-I complex on the target cell triggers the cytotoxic response. **2. Release of Cytotoxic Granules:** Upon activation and recognition of the target cell, cytotoxic T cells release specialized cytotoxic granules containing various molecules that mediate cell death. **3. Role of Perforin:** Perforin is a protein released by cytotoxic T cells. It forms pores in the target cell membrane, allowing the entry of other cytotoxic molecules into the target cell. Perforin facilitates the delivery of cytotoxic molecules into the target cell, enhancing its susceptibility to cell death. **4. Role of Granzymes:** Granzymes are protease enzymes released by cytotoxic T cells. Once inside the target cell, granzymes initiate the process of apoptosis by cleaving specific intracellular proteins. Granzymes can activate caspases, which are proteases involved in the apoptotic pathway, leading to the fragmentation of the target cell's DNA and the dismantling of its cellular structures. **5. Induction of Apoptosis:** The combined action of perforin and granzymes disrupts the integrity of the target cell's membrane and triggers apoptosis. Apoptosis is a controlled process of cell death that prevents inflammation and tissue damage associated with necrosis. **6. Role of Interferons and Tumor Necrosis Factor (TNF):** Interferons (IFNs) and tumor necrosis factor (TNF) are cytokines released by activated T cells and other immune cells. They play important roles in the immune response and contribute to the destruction of target cells: - **Interferons (IFNs)**: IFNs have antiviral properties and can inhibit viral replication within infected cells. They also enhance MHC-I expression on target cells, making them more recognizable by cytotoxic T cells. Additionally, IFNs can enhance the expression of Fas ligand (FasL) on cytotoxic T cells, which binds to Fas receptors on target cells, contributing to apoptosis. - **Tumor Necrosis Factor (TNF)**: TNF is a proinflammatory cytokine that plays a role in cell death and inflammation. It can activate apoptosis pathways in target cells and promote the destruction of infected or abnormal cells. In summary, activated cytotoxic T cells destroy target cells through a combination of mechanisms, including the release of perforin and granzymes, which induce apoptosis in the target cells. Interferons, perforin, granzymes, and cytokines like TNF work in concert to efficiently eliminate infected or abnormal cells while minimizing damage to surrounding tissues.

Activated T cells, specifically cytotoxic T cells (CD8+ T cells), play a crucial role in destroying target cells that are infected with intracellular pathogens, including viruses, or are otherwise abnormal. The destruction of target cells involves a combination of mechanisms aimed at inducing apoptosis (programmed cell death). Here's an overview of how activated T cells destroy target cells and the roles of key molecules such as interferons, perforin, granzymes, and tumor necrosis factor (TNF): **1. Recognition and Engagement:** Activated cytotoxic T cells recognize infected or abnormal target cells by interacting with antigens presented on the target cell surface via major histocompatibility complex class I (MHC-I) molecules. The interaction between the T-cell receptor (TCR) on the cytotoxic T cell and the antigen-MHC-I complex on the target cell triggers the cytotoxic response. **2. Release of Cytotoxic Granules:** Upon activation and recognition of the target cell, cytotoxic T cells release specialized cytotoxic granules containing various molecules that mediate cell death. **3. Role of Perforin:** Perforin is a protein released by cytotoxic T cells. It forms pores in the target cell membrane, allowing the entry of other cytotoxic molecules into the target cell. Perforin facilitates the delivery of cytotoxic molecules into the target cell, enhancing its susceptibility to cell death. **4. Role of Granzymes:** Granzymes are protease enzymes released by cytotoxic T cells. Once inside the target cell, granzymes initiate the process of apoptosis by cleaving specific intracellular proteins. Granzymes can activate caspases, which are proteases involved in the apoptotic pathway, leading to the fragmentation of the target cell's DNA and the dismantling of its cellular structures. **5. Induction of Apoptosis:** The combined action of perforin and granzymes disrupts the integrity of the target cell's membrane and triggers apoptosis. Apoptosis is a controlled process of cell death that prevents inflammation and tissue damage associated with necrosis. **6. Role of Interferons and Tumor Necrosis Factor (TNF):** Interferons (IFNs) and tumor necrosis factor (TNF) are cytokines released by activated T cells and other immune cells. They play important roles in the immune response and contribute to the destruction of target cells: - **Interferons (IFNs)**: IFNs have antiviral properties and can inhibit viral replication within infected cells. They also enhance MHC-I expression on target cells, making them more recognizable by cytotoxic T cells. Additionally, IFNs can enhance the expression of Fas ligand (FasL) on cytotoxic T cells, which binds to Fas receptors on target cells, contributing to apoptosis. - **Tumor Necrosis Factor (TNF)**: TNF is a proinflammatory cytokine that plays a role in cell death and inflammation. It can activate apoptosis pathways in target cells and promote the destruction of infected or abnormal cells. In summary, activated cytotoxic T cells destroy target cells through a combination of mechanisms, including the release of perforin and granzymes, which induce apoptosis in the target cells. Interferons, perforin, granzymes, and cytokines like TNF work in concert to efficiently eliminate infected or abnormal cells while minimizing damage to surrounding tissues.