How does cancer evade the immune system?
Knowing that the immune system eliminates almost all cancers as they develop, we now understand that every cancer that actually grows to be big enough to be found by scans or symptoms has somehow evaded the immune system at some point in its evolution. There are many different ways that cancer cells learn to evade the immune system, but as the immune system evolves and learns, so too does each cancer.
One of the ways that cancer cells can block the effects of the immune system is to use some of the checkpoint proteins as ways to turn off immune cells. For example, they can display one of the checkpoint proteins called PD-L1 on their surface. PD-L1 is mostly used by normal cells to signal that they are healthy and should not be destroyed by the immune system. In this case, the cancer cells are using a natural camouflage to trick the immune system into thinking that they are normal.