In the ideal world imagined by mid-twentieth-century biologists, the immune system constantly monitored the cells it encountered, pouncing on and destroying any aberrant ones. This monitoring work, called immunosurveillance, supposedly guaranteed that the body would be kept clear of intruders, or any kind of suspicious characters, cancer cells included. But as the century came to a close, it became increasingly evident that the immune system was not only giving cancer cells a pass and figuratively waving them through the checkpoints. Perversely and against all biological reason, it was aiding
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