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Unformatted text preview: NAME_____________________________________________________________TA__________________ SEC____ 1 7.012 Problem Set 5 FRIDAY October 29, 2004 Due to popular demand, and a congenial staff due to the recent performance of the Red Sox, this Problem set upon completion must be inserted into the box Problem sets will NOT be accepted late. Solutions will be posted on the web. Question 1 Viruses have evolved several mechanisms to evade recognition and elimination by the immune system. Our immune systems have likewise evolved ways to counteract these methods of evasion. a) What type(s) of cells of the immune system recognize virally infected cells? KILLER T cells! otherwise known as Cytotoxic T lymphocytes (CTLs) b) How do these cells do so? Explain briefly. The Killer T cell receptors (TCRs) bind specifically to viral protein epitopes presented on MHC class I molecules. The killer specific T cell upon recognition of this epitope/MHC I complex, secretes perforin which perforates or makes a hole in the infected cell, killing it. c) Besides viruses hiding-out from most of the immune system within cells, how might viruses further prohibit the previously mentioned immune cells from identifying them in host cells? ** Hint , our immune system has retaliated to this evasion technique with special immune cells called natural killer cells (NK cells for short). These NK cells recognize and kill cells that have low or no MHC class I expression at their surface. Viruses can block MHC I presentation at surface of host cells, or block transport, or down regulate MHC I so that the cell does not present antigens. It turns out NK cell activity is a little more sophisticated. NK cells do not kill cells that have MHC I receptors on their surface. Cells without MHC I receptors on their surface are candidates for killing by NK cells. But NK cells also need to be activated for killing by specific receptors on the target cell's surface. d) Knowing more about how NK cells function, suggest a way in which the virus may now evade NK cell recognition and thus avoid its subsequent demise. Viruses downregulate expression of host cell's specific activating receptors or prevent surface expression/transport. Or block downstream modulators of activator signals. e) In addition, all cells have mechanisms to protect themselves from double stranded RNA ( dsRNA ) viruses. What is this innate mechanism of protection? Briefly explain how it works. Answer obtained from WWW. The interferon + response: -blocks translation, -endonucleases, degrade RNA or can lead to apoptosis or death of the cell before the virus has a chance to replicate. Can render adjacent cells resistant to infection. Interferons are protein components of animal cells which are synthesized and excreted under a variety of stimuli and make other cells of the same species incapable of replicating virus. DeSomer and Cocito 1968 MIT Biology Department 7.012: Introductory Biology - Fall 200 Instructors: Professor Eric Lander, Professor Robert A. Weinberg, Dr. Claudette Gardel 2...
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