If cancer can spread so easily throughout your body, why can’t it spread from person to person? Aka why isn’t it contagious?
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One big reason they are not contagious is that we don’t shed cancer cells.
However it’s not entirely impossible, we know transmissible cancers in nature. Some exist in mammals, and many in molluscs.
There’s also one very rare type of or example of transmissible cancer in humans,but that’s transferred by organ transplant.
In science we use cancer models when we implant cancer cells into rodents, and those cancers grow in there. The animals have the chance to reject the cancer (i.e. the immune system wins). There’re factors define whether the cancer will be rejected, one is how agresssive the cancer is (there are ones known to be more aggressive), and how many cancer cells are implanted.
The natural ones we know are either transferred by bites or there’s one known genitalia tumor in dogs that is transferred as an STD. We humans bite each other rather seldomly so that pathway is rather locked.
I’m certain that if we wanted, we could artificially transfer cancers between humans via injection, but obviously nobody tried that. However, we are not that different from mice, so if we can do that in mice, I don’t see the reason why it would be impossible in humans.
I also believe it’s not entirely impossible that there were or will be some very rare natural cancer transmissions in the past or future of humanity. There would be perhaps more if we had habits to rub our cancers to each other or bite each other when having mouth cancer. I would say these kind of behavior patterns are rare.
Others mention that, as cancer cells come from individuals of different genetic background, the cancer cells would be rejected by the immune system just like transplanted organs. That’s partially true because there are cancer cells that can efficiently hide their identity from the immune system (we call it MHC downregulation but that’s beyond Eli5). This MHC thingy is the basis of transplant rejection, so if a cancer hides it, such cells would not be identified on the basis of coming from other individual. Still could be identified on the basis of lacking MHC but that’s another story and the immune system may fail that.
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