Natural killer (NK) cells are a type of lymphocyte that plays an important role in the innate immune response. Unlike T and B cells, which require activation and differentiation before they can recognize and respond to pathogens, NK cells are able to rapidly respond to infected or cancerous cells without prior exposure.
NK cells are able to recognize and kill infected or abnormal cells through several different mechanisms:
- Direct cytotoxicity: NK cells are able to directly kill infected or cancerous cells through the release of cytotoxic granules containing perforin and granzymes. Perforin creates a pore in the membrane of the target cell, allowing granzymes to enter and initiate apoptosis.
- Antibody-dependent cell-mediated cytotoxicity (ADCC): NK cells can also recognize and kill cells that are coated with antibodies through the Fc receptor on their surface. When the Fc receptor binds to the Fc portion of the antibody, it triggers the release of cytotoxic granules and leads to the destruction of the target cell.
- Production of cytokines: NK cells can produce a variety of cytokines, including interferon-gamma (IFN-γ), which can activate other immune cells and enhance the immune response to infections.
NK cells are also able to recognize and respond to cells that have downregulated or altered their expression of major histocompatibility complex (MHC) molecules, which are normally required for the recognition of infected cells by T cells. This allows NK cells to target cells that have evaded detection by the adaptive immune system.
NK cells play a critical role in the early stages of the immune response to infections and cancer, and deficiencies in NK cell function can lead to increased susceptibility to these diseases. On the other hand, overactivation of NK cells can lead to autoimmune diseases and other immune disorders. Therefore, the regulation of NK cell function is important for maintaining a proper balance between immunity and tolerance.