Cytotoxic T Cells and NK Cells – Killing Zombie Cells!!!
This video explains the mode of action of cytotoxic T cells (Tc cells) and Natural Killer cell (NK cells). Tc cells and NK cells come into action when our own cells become zombie cells, that means when our own cells turn against us. This could happen under two conditions; either the cell is virally infected or it has turned cancerous. In both cases, it is important that these cells are destroyed and thus there is a specific immune response by Tc cells or NK cells that is carried out to clear such zombie cells.
Natural Killer cells: They are the large granular lymphocytes and they show cytotoxic activity against tumor and virally infected cells. They can recognise the target cells in two ways. 1. NK cells employs NK cell receptors to distinguish abnormalities such as reduction in MHC I molecule or presence of unusual profile of surface Ag by tumor or virally infected cells. 2. It can also recognise target cells based on the fact that some tumor or infected cells are bound by Abs as a result of Immune Response because NK cells have receptors for Fc region of IgG and thus it will bind to these target bound Abs.
Tc Cells: They can recognise target cells by the presence of MHC I + peptide complex on their surface.
Mode of Action of NK cells and Tc cells: Both the cells have similar mode of action with one key difference. They both have presence of granules with only difference being that NK cells are constitutively cytotoxic; that means they always have granules present in them whereas Tc cells need to get activated before granules appear. These granules contains two important molecules: Perforin which is a pore forming protein and Granzymes which are serine proteases. Once NK cells or Tc cells recognises the target cells, it will be activated and exocytosis of granules will occur which will release the perforin into small spaces between the two cells (target cell and NK/Tc cell). This released perforin undergoes a Ca2+ induced conformational change that allows them to insert into the target cell membrane forming cylindrical pores.
These pores allow the granzymes entry to the target cells. Once granzymes enter the target cell, it initiates a cascade of reactions that result in the fragmentation of the target cell DNA into oligomers of 200 bp and thus it destroys the target cell.
Learn more about the Immune System – Army of Our Body here.