Natural killer cell is granular cytotoxic lymphoid cell that circulate in blood and migrate into the tissues and posses the ability to kill the target or pathogen infected cells.
LOCATION: Natural killer cells present on the blood circulation and can migrate into the tissue to find the target. NK cells constitute 5% to 10% of lymphocytes in human peripheral blood of human body.
PROPERTIES: Natural killer (NK) cells are functionally cytotoxic but different from the cytotoxic T-cell.
- They do not show any memory after the stimulation.
- They do not express antigen specific receptors.
- They express killer cell inhibitory receptors (KIRs)
- NK cells also express receptor for immunoglobulins or antibodies and can decorate themselves with antibodies that bind to the pathogen or proteins on the surface of the infected cells.
FUNCTION OF NK CELL: They are efficient cell killer and attack a variety of abnormal cells.
- These immune cells can recognize the pathogen infected cells and kill these cells.
- They can also recognize and destroy the tumor cells.
- Natural killer cells produce two proteins (enzymes) perforins and granzymes which form pores on the membrane of the target cells and leading to lysis of those cells.
SPECIAL FUNCTION: Natural killer cells express killer cell inhibitory receptors (KIRs) on their surface. It helps them to differentiate the healthy cells of our body from the pathogen infected cells.
Mode of Action: A. Normal healthy cells express MHC-I (Major Histocompatibility Complex) molecule on their surface. When the killer cell inhibitory receptors of NK cells bind to the MHC-I molecule of healthy cells, an intracellular signal is generated which result the inhibition of NK cell activation and do not destroy the healthy cells. This process helps the normal healthy cells to be alive.
B. When our healthy cell become infected by virus or other pathogens or become transformed into tumor cells, they do not express MHC-I molecule on their surface. Then they only contain antitumor or antiviral antibody (Ab) on their surface.
Natural killer cells also posses’ receptors (may be CD16) for immunoglobulins or antibodies which bind to the anti-tumor or antiviral antibodies present on the surface of the infected cells.
Then the NK cells get activated and start to release perforin and granzymes proteins which forms pores on the membrane of the target cells and ultimately causes lysis of the infected cells.
This whole process is mediated by antibody so, it is called Antibody Dependent Cell-Mediated Cytotoxicity (ADCC).