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.