Ans: B (Decreased...) IMMUNE SURVEILLANCE IN CANCER Robbins 8th/331-32Most cancers occur in persons who do not suffer from any overt immunodeficiency. It is evident then that tumor ceils must develop mechanisms to escape or evade the immune system in immunocompetent hosts.Several such mechanisms may be operative: They are-# Selective outgrowth of antigen-negative variants: During tumor progression, strongly immunogenic subclones may be eliminated.# Loss or reduced expression of MHC molecules: Tumor cells may fail to express normal levels of HLA class I molecules, thereby escaping attack by cytotoxic T cells. Such cells, however, may trigger NK cells# Lack of costimulatiom tumor cells may express peptide antigens with class I molecules, they often do not express costlmitlatory molecules. This not only prevents sensitization, but also may renderT cells anergic or, worse, cause them to undergo apoptosis. Because dendritic cells express high levels of co stimulatory molecules, it Is expected that such dendritic cell immunization will stimulate antitumor T cells.# Immunosuppression: Many oncogenic agents (e.g., chemicals and ionizing radiation) suppress host immune responses. Tumors or tumor products (like TGF-b) may also be immunosuppressive.# Antigen masking: The cell-surface antigens of tumors may be hidden, or masked, from the immune system by glycocalyx molecules, such as sialic acid-containing mucopolysaccharides. This may be a consequence of the fact that tumor cells often express more of these glycocalyx molecules than normal cells do.# Apoptosis of cytotoxic T cells: Some melanomas and hepatocellular carcinomas express Fas ligand. It has been postulated that these tumors kill Fas-expressing T lymphocytes that come in contact with them, thus eliminating tumor-specific T cellsImmunosuppressive cytokines (e.g., TGF-b)Fig: Mechanisms by which tumors evade the immune system