Tattoos are a divisive thing. Where some see beautiful body art, others see graffiti on skin. What everybody can agree upon is that tattoos are basically forever – and now we know why that's possible.
When you go under the needle, the ink is injected through its outer layer, the epidermis, into the inner layer called the dermis. Puncturing the skin makes your immune system kick into action - white blood cells called macrophages converge on the wound and the tattoo ink.
These macrophages gulp down the invading dye particles like they would any other foreign element – and it used to be thought that the longevity of these ink-swallowing cells was why tattoos are for keeps.
But that's not actually how it works, according to a new study by researchers from France's Centre d'Immunologie de Marseille-Luminy. It seems macrophages – unlike tattoos – don't necessarily live forever.
In experiments with tattooed mice, the team found that when they killed ink-bearing macrophages via toxic injection, the tattoos stayed perfectly in place on the mouse tails.
If it was the macrophages keeping the tattoo in place, this wouldn't be possible - killing those cells should dissolve or change the shape of the tattoo.
As it turns out, your tattoo designs are kept intact through a clever pass-it-on system within the skin tissue.
In the experiment, as macrophages died after their exposure to the toxin, they released their ink loads, which were shortly picked up by other immune cells in the surrounding tissue - and some of those cells were newly born.
This hand-over system can "undergo successive cycles of capture – release – recapture without any tattoo vanishing" the team explains in their paper.
It's this cell turnover process – "an immune system relay race baton", as Peter Hess at Inverse described it – which makes your tattoo stay right where you wanted it, even if you've long since changed your mind.
"When tattoo pigment-laden macrophages die during the course of adult life, neighbouring macrophages recapture the released pigments and ensure in a dynamic manner the stable appearance and long-term persistence of tattoos," one of the team, immunologist Sandrine Henri explains.
While the findings might make it seem like getting rid of tattoos is impossible – since legions of macrophages are only too happy to step in and hold you to your existing life choices – that might not be the case.
The researchers say the discovery of this hand-over system could actually improve the efficiency of laser-based tattoo removal techniques, by timing laser firing in sequence with the release of an engineered antibody-toxin.
"This approach would allow [removalists] to kill simultaneously all the macrophages that are laden with tattoo ink," the researchers told Inverse.
"Therefore, all the tattoo ink will be free within the dermis at the same time and accessible to the laser to break it [into] small pieces."
The findings are reported in the Journal of Experimental Medicine.