Veterinary Immunology and Serology

Natural Killer Cells

NATURAL KILLER CELLS (NK CELLS)

• They constitute 5-20% of mononuclear cells of blood and spleen but rare in other lymphoid organs.
• NK cells derived from blood or spleen can kill various target cells without additional activation.
• NK cells are neither T nor B-lymphocytes and do not express receptors like immunoglobulin or T cell receptors. They have activating receptors and inhibitory receptors. When both activating and inhibitory receptors are engaged, inhibitory receptors are dominant and NK cells are not activated. This mechanism prevents the killing of normal host cells. In case of infection, most often there is inhibition of class I MHC molecule expression and ligands for inhibitors receptor of NK cells are lost and they are active.
• NK  cells are a subset of large lymphocytes derived from bone marrow precursors with numerous cytoplasmic granules (large granular lymphocytes).
• NK cells also recognize antibody-coated targets. The effectors functions of NK cells are to kill the infected cells and activate macrophages to destroy the phagocytosed microbes.
• NK cells like T-cytotoxic cells have granules, which contain perforin (protein) that creates pores on the target cell membrane and the enzyme called granzymes, enter through the pores and cause apoptosis and death of target cells.
• NK cells kill, viruses, intracellular bacteria, and IFN-γ produced by NK cells help in phagocytosis by the macrophages. They participate in antibody dependent cell mediated cytotoxicity (ADCC).