Natural killer cells attacks which of the following cells -

Ans. is 'b' i.e., Cells which are not able to expres MHC I Null cells (Large granular lymphocytes) o Null cells are called so because they lack features of surface markers of both B and T lymphocytes. o 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. o They express Surface receptors for the Fe poion of IgG (CD-16) NCAM-I (CD56) o They proliferate in response to IL-2 o Many LGL express some lineage marker paicularly CD-8, CD-2. o LGLs arise in both bone marrow and thymic microenvironment. o Members of this group are : I. Antibody dependent cytotoxic cells (ADCC) o ADCC possess surface receptors for the Fc pa of Ig. o Capable of lysing or killing target cells sensitized with IgG. 2. Natural killer cells (NK Cells) Possess spontaneous cytotoxicity towards various target cells : - (i) Virus infected cells (ii) Malignant cells (iii) Transplanted foreign cells Cytotoxicity is not antibody dependent or MHC restricted o Activity is nonimmune as it does not require sensitization by prior antigenic contact. They have CD-56 and CD-16 on their surface o 1L-2 acts as a growth factor o Natural killer cell activity is augmented by interferon. o They release perforin which causes transmembrane pores through which TNF-I3 enters the cell and induce apoptosis. o NK cells are activated to LAK cells in the presence of IL-2 o Usually CD-3 negative but subset of NK cells are CD-3 positive called NK/T cells. o NK cells secrete TNF-a, GMCSF, perforin, IL-4. o NK cells do not kill normal cells. Why is it so? lets see : ? The NK cells express activating and inhibiting receptors. The functional activity of the NK cells is regulated by a balance between signal from these receptors. MHC class I molecules has inhibiting signal on NK cells and normal cell are not killed because inhibiting signals from normal MHC class I molecules overside activating signals. The ability of NK cells to kill target cells is inversly related to target cell expression of MHC class I molecule. If virus infection or neoplastic transformation reduces the expression of MHC I, inhibitory signals are interrupted and lysis of target cells occur. 3. Lymphokine activated killer (LAK) cells o These are NK - cells treated with IL-2 o Kill tumor cells more efficiently than unstimulated NK cents.