Don’t get me wrong, I’m glad I had no cause to get a cast as a kid ― but I have to admit I longed for the glamour of it.
After all, certain wounds (including the graphite from a pencil getting embedded into your skin, usually after a fight) are childhood rites of passage.
But while a broken leg will re-set, for some reason, pencil nubs seem to stay visible in the skin years after the offence occurs.
According to Dr Karan Rajan, who regularly shares his professional knowledge on TikTok and in his book, that’s because the injury can become what’s known as a “traumatic tattoo.”
A what now?
In a recent TikTok, the doctor explained that “If you’ve ever been stabbed by a pencil, chances are you’ve got a permanent pencil tattoo.”
These “traumatic tattoos” actually work in a similar way to the real thing, he says.
“When you’re ruthlessly attacked by a pencil, microscopic shards of graphite get stuck in the dermis,” or the thick layer of skin right underneath the top part we can touch, Dr Rajan said.
Your immune system recognises the graphite as a foreign object, leading your white macrophages (specialised white cells) to crowd around it in an attempt to absorb and then clear it from your body.
But the larger shards can be too big for the macrophages to do their job properly. In that case, “the threat isn’t eliminated but contained.”
“The macrophages surround the pencil shards and encapsulate it and hold it in place, immortalised in suspended animation” just below the top layer of skin, where you can still see them.
How is that like tattoos?
It’s the “exact same scientific process,” Dr Rajan shared.
It’s actually kind of a biological coincidence that the ink we use to mark line on paper has such a lasting impact on our skin too.
“Upon injury, immune cells flood the area and cause swelling, partially carrying away damaged cells and nanoparticles of ink,” Park Avenue Medicine writes.
“Only a tiny amount of the ink is actually removed from the initial site of the tattoo, while the rest remains in the dermis,” they add, which is why tattoos fade.
A 2018 study found that there’s a release-recapture cycle at play here: the macrophage engulfs the ink, which is trapped in a vacuole, where it remains unharmed.
The macrophage dies off before it has a chance to remove the ink ― so it re-releases it, where it’s swallowed by another macrophage. This keeps it in place.
Who knew there was so much science behind your primary school scuffle?