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  • Alex Cloherty

Ink eaters

Over Christmas, my little sister Michaela (who is now a bona fide tattoo artist and co-owner of the Cowpoke Collective) gave me some new ink.

While Michaela was preparing to give me the new tattoo, I watched her sanitise her workspace in much the same way as I prepare my own working area in the lab. We even use some of the exact same tools, like ethanol to decontaminate surfaces and plastic wrap as sealant. So of course, this got me thinking about how our two career paths intersect.


It’s not just our tools of work that are similar. The functioning of the immune system is at the very root of the reasons behind why tattoos can last a lifetime.


Our skin is one of our body’s most important barriers, and as such it is constantly encountering new and possibly problematic microbes. In an effort to be prepared for any unexpected onslaught, our skin is absolutely packed full of immune cells. I wrote about one type of skin immune cell, the Langerhans cell, in the past, but today we’ll focus on a different type of immune cell: the macrophage.


I’m especially partial to macrophages because they, like me, love snacks. Their name means ‘mega eater’, which is a good way to remember that their major function is to eat up anything that penetrates the skin and could cause harm, be that bacteria, viruses, dying human cells, or even non-living things like wood or metal. These little immune cells will even try to eat up invading things that are thousands of times their size - they’ll cluster around macroscopic slivers and try in vain to gobble up the wood until you manage to pull it out with some tweezers. They are like valiant little cookie monsters that gorge themselves in the name of your immune defense system.


But although macrophages are constantly eating, this cannot go on forever. Eventually, these hungry heroes will die. But, they are quickly replaced by new cells that squeeze out of your blood vessels and into your skin, and then ‘grow up’ to become brand new hungry hungry macrophages.


Now, when macrophages die, it can happen that they release the contents of their guts into their surroundings. It’s a messy way to go: as they bite the dust, they barf. At best, it’s just a wee vomit, but at worst, this can mean that macrophages accidentally spew out dangerous microbes that they’ve ingested as they kick the bucket. And that’s why when one macrophage dies, it is pretty common for other macrophages to come and clean up the mess their buddy made - just in case there’s something dangerous hiding in its corpse.


And as it so happens, tattoo artists like my sister have for centuries piggy-backed on this protective immune mechanism to inkily immortalise their art on the skin of their fellow humans.


When a tattoo artist’s needle pierces the skin of their client, it deposits ink into the dermis, the layer of skin underneath the outward-facing epidermis. And the dermis is where macrophages live.


Macrophages flag the ink as a threat, and so they do what they do best: they eat it up. However, there’s a catch.


Normally, macrophages are very good at digesting and breaking down whatever they’ve eaten up, thanks to specialised acids and proteins in their cellular digestive system. But the ink stumps them. Macrophages can’t digest it, and so the ink and the macrophage enter a sort of stalemate. The macrophage wants to digest the ink, but it simply can’t manage to break it down. But, the macrophage also doesn’t want to spit out the ink, just in case it is dangerous to the human that it is part of. So instead, the ink stays trapped within the macrophage, and the macrophage sits in place in the skin, keeping its cargo contained and immobilised. This would be bad news for a bug (from the bug’s perspective, at least) - but it’s exactly what we humans want when we get a tattoo: for it to stay in place, not going anywhere!


But, remember, macrophages don’t live for as long as we humans do. Eventually macrophages die and release their cargo. So why doesn't the tattoo disappear when all of the macrophages that first gobbled up the ink die out?


Well, because the next generation of macrophages see to it that that the ink remains gobbled up for good. When the ink-containing macrophages die out, the next generation of macrophages, which you might remember come from cells that squeeze out of your bloodstream and into your skin to develop into new macrophages, eat up all of the ink that explodes out of the old, dying macrophages. In other words, a “capture-release-recapture” cycle of ink-eating by macrophages keeps the tattoo forever in place. This is also why tattoos are so very hard to get rid of. Even if you succeed in killing the macrophages currently holding onto your tattoo ink, to really get rid of the tattoo for good, you also have to make sure that the ink is dispersed so that new macrophages won’t quickly gobble it up as the old ones puke out the pigment.


So whether you love or hate tattoos, now you know: macrophages are to blame.


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Thanks for reading another Microbial Mondays article! If you liked this article about macrophages, be sure to stay tuned for the guest blog series by Merel Sijbranda in mid-February, which will dig much deeper into the roles of macrophages not in tattoo longevity, but in infectious disease.

Finally, this paper was the primary source material for this article. Thanks to the authors for sharing their interesting and informative work, and without a paywall!


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