(╯°□°)╯︵ ┻━┻

Can't enjoy watermelon; can't enjoy apples; what next.

You cannot build up an immunity to cyanide. The mechanism of action for it to act as a poison is that it directly limits aerobic respiration on a molecular level. To put that more simply, it stops oxygen from getting where it needs to go and effectively stops your cells from "breathing", ultimately stopping your mitochondria from functioning. To put that in a more complex way, it binds to part of an enzyme called cytochrome c oxidase that is essential to ATP synthethis and prevents the reaction from occurring.

Horrifyingly, cyanide poisoning cannot be fixed by simply giving the body higher concentrations of oxygen to breathe in. It doesn't prevent your repository system from uptaking oxygen. Instead it prevents your individual cells from using the available oxygen.

While it is sort of technically possible in the case of cyanide for the liver to produce more rhodanese (an enzyme that specifically reduces the toxic effects of cyanide in the body) the reaction is dependent on sulfur. Perhaps if you ate foods like eggs in very large quantities and also regularly ate larger amounts of cyanide you could maintain a state of immunity to doses larger than what would normally kill a person but I can't find any evidence of someone stupid enough to test that. Regardless this immunity is in the form of getting your liver to simply clear the cyanide from your system faster and does nothing to stop the underlying cellular asphyxiation.

As an aside Arsenic which was also mentioned earlier interrupts mitochondrial function as well, but in this case it takes the place of Phosphorus (The P in ATP) and the resulting molecule is not able to react correctly. While not a perfect comparison you can think of cyanide as starving your cells of oxygen while arsenic starves them of useful fuel. Neither of these are something you can build an immunity to.

Thats an old wives tale methinks. The amount ingested via apple seeds is so small that it is unlikely to generate enough response for immunologic or epigenetic adaptations to be unaffected. Even if it does result reduce effects in any sense it would likely be overwhelmed in any purposeful poisoning.

Sadly i don't know about the immunity part. While i have dug through the depths of the internet to find out how many and which sort of seeds are required for lethal dose and even then the estimate range is huge, lovest end 10-20kg up to 100kg worth of apples and no mention of which sort. I haven't looked into the immunity part.

In addition the seeds need to be ground up, chewing might not even be enough to break up the seeds well enough for the digestive system. Otherwise those will pass through fully intact.