NK Cells - Direct Killing PDF

## NK Cells - Direct Killing NK cells directly kill target cells via **Direct Killing**. - NK cells bind to target cells which causes death. - This death is **apoptotic** (programmed cell death). - Phagocytosis is not effective against viruses, thus the cell must activate NK cells. - Cell death...

## NK Cells - Direct Killing NK cells directly kill target cells via **Direct Killing**. - NK cells bind to target cells which causes death. - This death is **apoptotic** (programmed cell death). - Phagocytosis is not effective against viruses, thus the cell must activate NK cells. - Cell death is considered **sterile**, as no substances are released outside the cell. ### NK Cell's Killing Granules - NK cells have killing granules that are released upon contact with a target cell. - Granules contain **perforin** and **granzyme**: - **Perforin** creates a pore in the target cell membrane. - **Granzyme** is a protease that is injected through the pore. - Granzyme cleaves caspases, which activate apoptosis. - If NK cell detaches from the target cell, apoptosis will continue as NK cell has already released the destructive substances. ### Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC) - NK cells express Fc receptors (FcyR, CD16), which recognize antibodies. - When antibodies bind to the target cell, NK cell can recognize them. - This activates NK cell to kill the target cell by releasing its granules. - ADCC takes longer than direct killing, as one NK cell can kill only one target cell at a time. ### Other Killing Mechanisms - Mast cells, basophils, eosinophils are capable of releasing their killing granules outside the cell. - This is considered a "dirtier" method, as the killing substances are not directly injected into the target cell. - These cells can be triggered for this mechanism via antibodies. - This is especially useful against large pathogens (ticks, worms, etc.)