Why Do Immune Cells Leave Cancer Alone?

Siddhartha Mukherjee says immune cells leave cancer alone because the cancer cells that survive display dozens of surface flags telling T cells and B cells they are normal. Siddhartha Mukherjee says a checkpoint is a gene or a flag that a normal cell floats on its surface, saying it is normal and should be left alone.

The flags are why people do not live with desperate, terrifying autoimmunity. The body has enough T cells to eat itself alive. People who have mutations in some checkpoints are consumed by T cells. Diabetes type 1 is a T cell autoimmune disease that eats the pancreas alive. Lupus, another immune-cell disease, eats the body alive. In scleroderma, a patient is eaten alive by their own autoimmune cells. Healthy cells carry not one flag and not two flags but dozens of flags, telling a B cell and a T cell they are normal, so the immune cells do not eat them, kill them, or start an autoimmune response.

Cancer cells figure out the same protection. Mukherjee calls the picture anthropomorphic, then gives the evolutionary version: the cancer cells that survive are the cancer cells able to put up the flags.