תאי NK - הריגת תאים ישירה

NK Cells - Direct Killing

• NK cells kill infected or cancerous cells through a direct killing mechanism.
• This process is distinct from phagocytosis, as it doesn't involve engulfment.
• NK cells recognize and bind to target cells, releasing granules containing perforin and granzymes.
• Perforin creates pores in the target cell membrane.
• Granzymes enter the target cell, activating apoptosis (programmed cell death).
• This ensures a sterile killing process, without releasing harmful substances to other cells.
• Even if the NK cell detaches, the target cell will die, due to the internalized enzymes.

Perforin and Granzyme

• Perforin is a protein that creates channels (pores) in the target cell's membrane.
• Granzyme is a protease enzyme, which activates apoptosis when inside the target cell.
• Both are crucial proteins inside the granules of NK cells, released to kill cells.

Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

• Antibodies can mark target cells for NK cell destruction.
• NK cells have receptors (Fc receptors) that recognize the antibodies bound to target cells.
• This cross-linking of Fc receptors activates the NK cell to kill the target cell through granule release.
• ADCC is a slower process compared to direct killing, requiring one NK cell per target cell.

Other Cell Killing Mechanisms

• Macrophages, eosinophils, mast cells, and basophils can also kill pathogens.
• These cells release granules to the extracellular environment – indirectly.
• This method is often used for larger pathogens like parasites.
• Antibody binding to the target cell and these cells' receptors triggers granule release.
• Allergy reactions often involve this indirect method, as well as innate immunity activation against allergens/irritants.