Cancer immune escape

Cancers are not viruses, however,and they don’t evade CTL in the same ways. One difference is that cancers evolve from square one each time. When you’re infected by a virus, that virus is the end-product of an unbroken chain of evolution that goes back millions or billions of years;it’s had time to evolve its own specialized molecules, which may or may not have been based on the host genome at some time but is now generally a standalone, distinct gene. Cancers don’t have that history. Each individual cancer arose independently within you,and it only has your lifespan5 in which to experiment with immune evasion.
A part of this selective destruction can lead to cancer immune escape. I’ve talked about the way chronic viruses (like hepatitis C and HIV) alter their protein sequences in such a way as to mutate MHC class I epitopes, so that immunodominant CTL no longer recognize infected cells. Cancers are much like chronic infections and they too mutate immunodominant epitopes, so that CTL can no longer recognize them.8 This is, obviously, a real concern in cancer immunotherapy trials.
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