Natural killer(NK) cells are large granular lymphocytes. They are Non-phagocytic cells. The granules of natural killer cells contains preformed biologically potent molecules.
• The concentration of NK cells of total Lymphocytes is 5% to 10%.
• Some of these molecules has capability to form pores in the membrane of target cells, Resulting in the lysis of the cell.
• Some other molecules induce apoptosis of the target cell.
Natural killer cells play role in defense against both intracellular and extracellular pathogens.
NK cells Defense against intracellular Pathogens
• When a cell is infected with pathogens, the cell secretes type 1 interferon.
• These interference bind to the interferon receptors on the natural killer cells and as a result natural killer cells get activated.
• They proliferate and differentiate into Effected NK cells. These effector NK cells has capability to recognise the cells which are infected by pathogens.
• For this they use specific receptors present on the surface which recognise the altered protein on the surface of Infected cells.
• NK cell makes contact with the infected cells through these receptors.
• Once contact has been established the granules of the effector NK cell fused with the plasma membrane of antigen and these granules are short out onto the outer surface of the target cell.
• These granules contain biologically potent molecules most of which are toxic proteins also known as cytotoxins. E.g. Perforin.
- Perforin entered into the plasma membrane of the target cell and create perforations in the membrane. This leads to the lysis of the cells or cytolysis.
- Another important toxic protein is Granzyme. It induce the target cell to undergo apoptosis.
• Thus an T-cells result in killing of infected cells either by cytolysis or by apoptosis or program cell death.
NK cells Defense against Extracellular Pathogens
• NK cells are non-phagocytic cells, so they cannot kill extracellular pathogens directly.
• NK cells secrete interferon(γ) that is type 2 interferon.
• This interferon acts as macrophage activation factor. that is interference gamma, once released by the natural killer cells bind to specific receptors on the macrophages.
• As a result macrophage becomes more activated and more efficient in phagocytosis and destruction of the pathogens.
Natural killer cells provide defense against both intracellular and extracellular pathogens. In case of intracellular pathogens and T cells are directly involved, but they provide defense against extracellular pathogen indirectly.
