Natural killer （NK） cells are bone marrow derived, large granular lymphocytes, comprising approximately 10% to 20% of the mononuclear cell fraction in normal peripheral blood. They form a part of the first line defense mechanism against tumoural and viral spreading^1-4 Unlike T and B cells, NK cells do not require gene rearrangement for assembly of their receptor genes; rather, NK cells discriminate potential target cells based on the levels of self major histocompatibility complex （MHC） class I expression on such cells^5.6 There are two kinds of NK cell receptors^2.7.8 Inhibitory receptors recognize MHC class I molecules and deliver a downregulatory signal that inactivates the lytic machinery of NK cells. Stimulatory receptors expressed by NK cells deliver an activation signal.
JIANG Yong-jun ZHANG Zi-ning DIAO Ying-ying GENG Wen-qing DAI Di LIU Jing WANG Ya-nan ZHANG Min HAN Xiao-xu SHANG Hong
