Natural killer cell is -: (A) MHC restricted, (B) Antibody dependent, (C) Null cells, (D) B-lymphocytes
Answer is C. Ans. is 'c' i.e., Null cells Null cells (Large granular lymphocytes or LGLIo Null cells are called so because they lack features of surface markers of both B and T lymphocytes. The account for 5 to 10% of peripheral blood lymphocytes.o They are also called "large granular lymphocytes (LGL)"as they contain large azurophilic cytoplasmic granules. They express CD 16 and CD56 markers. They proliferate in response to IL-2.o LGLs arise in both bone marrow and thymic microenvironment. In contrast to T-cells, they are not MHC restricted.o Various types of LGLs are:-1) Antibody dependent cytotoxic cells (ADCC)# These cells kill/lyse target cells which are sensitized with IgG.2) Natural killer cells (NK cells)# These cells possess cytotoxic activity against virus infected cells, tumor cells and transplanted foreign cells.# Cytotoxicity of NK cells is neither antibody dependent nor MHC restricted. Activity is nonimmune as it does not require antigenic stimulation.# NK-cells are positive for CD16 and CD56.# NK cells are usually negative for CD3, but a subset is positive for CD3 called NK/T-cells.# IL-2 acts as a growth factor for NK cells. NK cells activity is augmented by interferon. NK cells secrete peroforin (resembles comlement C9) which causes transmembrane pores through which cytotoxic factors (e.g. TNF-b) enters the cells and induce apoptosis.o Virus infected cells and tumor (cancer) cells are killed by NK cells and cytotoxic T-cells.3) Lymphokine activated killer (LAK) cells# These are NK cells which are activated by IL-2. These cells kill tumor cells more efficiently than unstimulated NK cells.