Chapter 21 - Section 21.4 - Cellular Immunity - Apply What You Know - Page 829: 1

Cytotoxic T cells (CD8+ T cells) and natural killer (NK) cells are both types of immune cells that play roles in eliminating infected or abnormal cells from the body. While they share some similarities in their cytotoxic activities, they also have distinct characteristics that set them apart. Here's how cytotoxic T cells are similar to and different from NK cells: **Similarities:** 1. **Cytotoxic Activity:** Both cytotoxic T cells and NK cells are capable of recognizing and killing target cells. They are part of the body's defense against infected cells, cancer cells, and cells that have been otherwise compromised. 2. **Cell Surface Receptors:** Both cell types possess receptors on their surfaces that allow them to detect abnormalities in target cells. These receptors enable the immune cells to distinguish between healthy and damaged cells. 3. **Perforin and Granzymes:** Both cytotoxic T cells and NK cells use cytotoxic molecules like perforin and granzymes to induce cell death in target cells. Perforin creates pores in the target cell's membrane, allowing granzymes to enter and trigger apoptosis (programmed cell death). **Differences:** 1. **Specificity:** - **Cytotoxic T Cells:** Cytotoxic T cells are highly specific and require prior sensitization to a particular antigen. They recognize antigens presented by infected or abnormal cells through their T cell receptors (TCRs). - **NK Cells:** NK cells have a broader specificity and do not require prior sensitization. They can recognize cells with reduced levels of self-MHC molecules or cells expressing stress-induced molecules on their surfaces. 2. **Major Histocompatibility Complex (MHC) Recognition:** - **Cytotoxic T Cells:** Cytotoxic T cells primarily recognize antigens presented on MHC class I molecules on the surface of target cells. - **NK Cells:** NK cells can recognize cells with altered MHC class I expression (missing self) or with altered MHC class I/peptide complexes (induced self), allowing them to detect cells that may be infected or transformed. 3. **Activation:** - **Cytotoxic T Cells:** Cytotoxic T cells require interaction with antigen-presenting cells (APCs) to become activated. They need to receive co-stimulatory signals and TCR engagement. - **NK Cells:** NK cells do not require prior sensitization or antigen presentation. They are activated by the balance between activating and inhibitory signals received from cell surface receptors. 4. **Memory:** - **Cytotoxic T Cells:** Cytotoxic T cells can develop memory, allowing for a rapid and specific response upon re-exposure to the same antigen. - **NK Cells:** NK cells do not generate memory responses like cytotoxic T cells. Their activity remains more innate and immediate. In summary, cytotoxic T cells and NK cells both possess cytotoxic capabilities, but their mechanisms of activation, specificity, and modes of recognition differ. Cytotoxic T cells require antigen-specific activation, while NK cells provide a broader and more rapid response to abnormal cells based on altered cell surface markers.

Cytotoxic T cells (CD8+ T cells) and natural killer (NK) cells are both types of immune cells that play roles in eliminating infected or abnormal cells from the body. While they share some similarities in their cytotoxic activities, they also have distinct characteristics that set them apart. Here's how cytotoxic T cells are similar to and different from NK cells: **Similarities:** 1. **Cytotoxic Activity:** Both cytotoxic T cells and NK cells are capable of recognizing and killing target cells. They are part of the body's defense against infected cells, cancer cells, and cells that have been otherwise compromised. 2. **Cell Surface Receptors:** Both cell types possess receptors on their surfaces that allow them to detect abnormalities in target cells. These receptors enable the immune cells to distinguish between healthy and damaged cells. 3. **Perforin and Granzymes:** Both cytotoxic T cells and NK cells use cytotoxic molecules like perforin and granzymes to induce cell death in target cells. Perforin creates pores in the target cell's membrane, allowing granzymes to enter and trigger apoptosis (programmed cell death). **Differences:** 1. **Specificity:** - **Cytotoxic T Cells:** Cytotoxic T cells are highly specific and require prior sensitization to a particular antigen. They recognize antigens presented by infected or abnormal cells through their T cell receptors (TCRs). - **NK Cells:** NK cells have a broader specificity and do not require prior sensitization. They can recognize cells with reduced levels of self-MHC molecules or cells expressing stress-induced molecules on their surfaces. 2. **Major Histocompatibility Complex (MHC) Recognition:** - **Cytotoxic T Cells:** Cytotoxic T cells primarily recognize antigens presented on MHC class I molecules on the surface of target cells. - **NK Cells:** NK cells can recognize cells with altered MHC class I expression (missing self) or with altered MHC class I/peptide complexes (induced self), allowing them to detect cells that may be infected or transformed. 3. **Activation:** - **Cytotoxic T Cells:** Cytotoxic T cells require interaction with antigen-presenting cells (APCs) to become activated. They need to receive co-stimulatory signals and TCR engagement. - **NK Cells:** NK cells do not require prior sensitization or antigen presentation. They are activated by the balance between activating and inhibitory signals received from cell surface receptors. 4. **Memory:** - **Cytotoxic T Cells:** Cytotoxic T cells can develop memory, allowing for a rapid and specific response upon re-exposure to the same antigen. - **NK Cells:** NK cells do not generate memory responses like cytotoxic T cells. Their activity remains more innate and immediate. In summary, cytotoxic T cells and NK cells both possess cytotoxic capabilities, but their mechanisms of activation, specificity, and modes of recognition differ. Cytotoxic T cells require antigen-specific activation, while NK cells provide a broader and more rapid response to abnormal cells based on altered cell surface markers.