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Learning Outcomes:
i. Define natural killer (NK) cells and explain their role in the immune system.
ii. Describe the structure of NK cells and the surface molecules that distinguish them from other immune cells.
iii. Explain how NK cells recognize and kill cells infected with microbes, highlighting the role of activating and inhibitory receptors.
iv. Discuss the role of NK cells in cancer immunosurveillance, identifying and eliminating cancer cells.
v. Recognize the significance of NK cells in innate immunity and their potential therapeutic applications.
i. Natural Killer (NK) Cells: Immune System Sentinels
Natural killer (NK) cells are a distinct subset of lymphocytes, a type of white blood cell, that play a crucial role in innate immunity. Unlike other immune cells that require prior exposure to a pathogen, NK cells can directly recognize and kill infected cells without prior sensitization.
ii. Structure of NK Cells: Distinguishing Features
NK cells possess a unique set of surface molecules that distinguish them from other immune cells:
CD16: NK cells express CD16, a receptor that binds to the Fc portion of IgG antibodies, enabling NK cells to mediate antibody-dependent cellular cytotoxicity (ADCC).
NKG2D: NK cells also express NKG2D, a receptor that recognizes stress-induced ligands expressed on cells under stress, such as virus-infected cells and cancer cells.
Inhibitory receptors: NK cells express a variety of inhibitory receptors that recognize self-antigens, molecules expressed on healthy cells. These receptors prevent NK cells from attacking healthy cells.
iii. Recognizing and Killing Infected Cells: The NK Cell Response
NK cells continuously scan the body for infected cells:
Activating receptors: Upon encountering an infected cell, NK cells engage activating receptors, such as NKG2D, which bind to stress-induced ligands expressed on the infected cell.
Inhibitory receptors engagement: Simultaneously, NK cells also engage inhibitory receptors that bind to self-antigens expressed on the infected cell.
Activation or inhibition: The balance between activating and inhibitory signals determines whether the NK cell will be activated to kill the infected cell or remain inactive.
iv. Cancer Immunosurveillance: NK Cells against Cancer
NK cells play a critical role in cancer immunosurveillance, the constant monitoring of the body for cancer cells:
Cancer cell recognition: NK cells can recognize and kill cancer cells due to the reduced expression of self-antigens on cancer cells, allowing activating receptors to dominate over inhibitory receptors.
Elimination of cancer cells: NK cells directly kill cancer cells through a variety of mechanisms, including perforin-mediated pore formation and death receptor activation.
Significance of NK Cells: Sentinels of Health
NK cells are essential components of innate immunity and have potential therapeutic applications:
Innate immune defense: NK cells provide rapid and non-specific protection against a wide range of infections, including viral infections and bacterial infections.
Cancer immunosurveillance: NK cells play a crucial role in eliminating cancer cells, contributing to cancer prevention and treatment.
Therapeutic potential: NK cells are being investigated as potential therapeutic agents for cancer immunotherapy, where they are engineered to enhance their anti-cancer activity.
Natural killer (NK) cells are sentinels of the immune system, constantly scanning the body for infected cells and cancer cells. Their ability to recognize and kill these cells without prior sensitization makes them a critical component of innate immunity. Understanding the mechanisms of NK cell activation and their role in cancer immunosurveillance highlights their importance in maintaining overall health and the potential of NK cell-based therapies for cancer treatment.
