Final answer:
Natural Killer (NK) cells are an essential part of the innate immune system, targeting cells infected by viruses and intracellular bacteria, and inducing their apoptosis through mechanisms involving the altered expression of MHC class I molecules and the action of perforin and granzymes.
Step-by-step explanation:
Natural Killer (NK) cells are crucial components of the body's innate immune system. These lymphocytes specialize in combating infections, particularly from viruses and intracellular bacteria. NK cells have an innate ability to recognize cells that are infected with intracellular pathogens, largely through the altered expression of major histocompatibility complex (MHC) class I molecules. If a healthy cell's MHC class I molecules are intact and normal, the NK cell will leave it alone. However, infected or cancerous cells often display a different pattern of these molecules, alerting the NK cells to a problem.
Once an NK cell identifies an unhealthy cell, it releases cytotoxic granules composed of perforin and granzymes. Perforin punches holes in the target cell's membrane, allowing granzymes to enter and induce apoptosis or programmed cell death in the infected or abnormal cell. Following apoptosis, phagocytic cells then remove the cellular debris. This process is a crucial line of defense in controlling potential infections and preventing the progression of cancer.
NK cells can also employ another apoptotic mechanism via the fas ligand, which binds to its receptor on an infected cell and initiates cell death. This multi-faceted approach by NK cells showcases their importance in maintaining the body's health by patrolling for and eliminating cells that pose a risk due to infection or tumorigenic transformation.