Malignant tumors are sneaky. They tend to fly under the immune system’s radar because, unlike invaders such as viruses or bacteria, cancer cells arise from normal cells, so they bear many of the same familiar molecules on their surface.

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Once tumors are established, they become even more adept at hiding out from the immune system. They might cloak themselves in proteins to block immune cells from entering them or undergo genetic changes to further reduce the chance that disease-fighting cells will notice anything is amiss.