Chapter 21 - Section 21.2 - Study Guide - Assess Your Learning Outcomes - Page 844: 11

Natural Killer (NK) cells are a type of cytotoxic lymphocyte that play a critical role in the immune system's defense against virus-infected cells and cancer cells. They are part of the innate immune system and are particularly effective at recognizing and eliminating abnormal cells without the need for prior sensitization. Perforins and granzymes are two key molecules used by NK cells in their defense mechanisms. **Actions of Natural Killer (NK) Cells:** 1. **Recognition of Abnormal Cells:** NK cells are capable of recognizing cells that display abnormal surface features, such as downregulated MHC class I molecules. Normal cells express MHC class I molecules that present self-antigens. In contrast, many virus-infected cells and cancer cells downregulate MHC class I molecules to evade immune recognition. NK cells monitor these changes and target cells lacking or displaying altered MHC class I molecules. 2. **Release of Cytotoxic Granules:** Upon recognizing a target cell, NK cells release cytotoxic granules containing two key components: perforins and granzymes. **Roles of Perforins and Granzymes in Defense:** 1. **Perforins:** Perforins are proteins that form pore-like structures in the target cell's plasma membrane. These pores allow water and ions to enter the cell, leading to osmotic swelling and eventual cell lysis. Perforins also facilitate the entry of granzymes into the target cell. 2. **Granzymes:** Granzymes are proteases, or enzymes that cleave proteins, released by NK cells. Once perforins create pores in the target cell's membrane, granzymes can enter the cell's cytoplasm. Once inside, granzymes initiate a series of events that induce apoptosis, a programmed cell death process. Granzymes target various cellular components, including caspases, which are key regulators of apoptosis. Together, the actions of perforins and granzymes lead to the destruction of the target cell. By inducing apoptosis, NK cells prevent the spread of viral infections and eliminate cancer cells that could otherwise evade immune detection. The ability of NK cells to recognize and destroy abnormal cells without prior sensitization is a crucial aspect of the body's defense against infections and malignant growth.

Natural Killer (NK) cells are a type of cytotoxic lymphocyte that play a critical role in the immune system's defense against virus-infected cells and cancer cells. They are part of the innate immune system and are particularly effective at recognizing and eliminating abnormal cells without the need for prior sensitization. Perforins and granzymes are two key molecules used by NK cells in their defense mechanisms. **Actions of Natural Killer (NK) Cells:** 1. **Recognition of Abnormal Cells:** NK cells are capable of recognizing cells that display abnormal surface features, such as downregulated MHC class I molecules. Normal cells express MHC class I molecules that present self-antigens. In contrast, many virus-infected cells and cancer cells downregulate MHC class I molecules to evade immune recognition. NK cells monitor these changes and target cells lacking or displaying altered MHC class I molecules. 2. **Release of Cytotoxic Granules:** Upon recognizing a target cell, NK cells release cytotoxic granules containing two key components: perforins and granzymes. **Roles of Perforins and Granzymes in Defense:** 1. **Perforins:** Perforins are proteins that form pore-like structures in the target cell's plasma membrane. These pores allow water and ions to enter the cell, leading to osmotic swelling and eventual cell lysis. Perforins also facilitate the entry of granzymes into the target cell. 2. **Granzymes:** Granzymes are proteases, or enzymes that cleave proteins, released by NK cells. Once perforins create pores in the target cell's membrane, granzymes can enter the cell's cytoplasm. Once inside, granzymes initiate a series of events that induce apoptosis, a programmed cell death process. Granzymes target various cellular components, including caspases, which are key regulators of apoptosis. Together, the actions of perforins and granzymes lead to the destruction of the target cell. By inducing apoptosis, NK cells prevent the spread of viral infections and eliminate cancer cells that could otherwise evade immune detection. The ability of NK cells to recognize and destroy abnormal cells without prior sensitization is a crucial aspect of the body's defense against infections and malignant growth.