Nk cell

NK cells

 Natural killer (NK) cells are large granular lymphocytes derived from the common lymphoid progenitor cells (lymphoblasts).

 NK cells constitute 5%-10% of lymphocytes in human peripheral blood.

 NK cells are cytotoxic; small granules in their cytoplasm contain proteins such as perforin and proteases known as granzymes, that will trigger either apoptosis or cell lysis in an abnormal cell.

 Defensins, an antimicrobial secreted by NK cells, directly kills bacteria by disrupting their cell walls.

 NK cells differentiate and mature in thymus, spleen, bone marrow, lymph nodes and tonsils.

NK cells provide an early response to virus infection

 

Types of NK cells

On the basis of the abundance of the CD56 glycoprotein on the cell surface and their cytotoxic potential NK cells are divided into two subpopulations:

 CD56dim NK cells:

 It comprises upto 90%, of NK cells and is considered the most cytotoxic subset.

 It binds to the FC portion of IgG antibodies and mediate antibody dependant cell-mediated cytotoxicity (ADCC) of antibody-bound target cells.

 Expression of granzyme B and perforin is also high in this subset compared with CD56bright NK cells.

CD56bright NK cells:

 This subtype comprises up to 10% of NK cells.

 This subset is known as the cytokine producing subset of NK cells, and rapidly produces cytokines and chemokines including IFNγ, TNFα, GMCSF after activation.

 A specialized population of NK cells are found in woman’s uterus, called uterine NK cells (uNK).

 They are essential for forming the placenta.

 The role of the uterine NK cells is not to kill cells and develop inflammation but to cooperate with fetal trophoblast cells in enlarging maternal blood vessels so that they have the capacity to supply the placenta, and thus the growing fetus, with sufficient oxygen and nourishment throughout pregnancy.

 

Functions

NK cells kill cells infected with virus by blocking replication of virus.

 It maintains and even increase the inflammation in the infected tissue by secreting inflammatory cytokines such as interleukin (IL)-2, IL-12, IL-15 and interferon gamma.

 NK cells also kills tumor cells, they recognizes cells that lack expression of major histocompatibility complex (MHC) class I antigens.