Tattoo'd ankle

Why Tattoos Stay Put

Tuesday, May 28, 2024

After years of internal debate, I finally got my first tattoo. He’s a small lizard, with a topological map design inside, scaling the outer edge of my left ankle. His fine lines and tasteful detail are the work of my incredibly talented cousin and artist, Ally Easter (ig: alzster, look them up!).

While I lay prone on the hospital-like bed, my mouth turned up in a forced smile, pretending like I barely noticed the incessant cat scratches at my ankle, I wondered what was going on down there. How exactly was this lizard going to stay with me for life? I wanted to know the science behind it all, more specifically, the immunology. I’m a third-year graduate student studying the immune system, and at this point my research career, I love a little morsel of immunology that isn’t my thesis project. 

Tattoo guns are like little electrified hammers. At the tip of the gun is a collection of needles of varying thickness, kind of like the hairs of a paintbrush, that punch down and up, into and out of the skin. Our skin is our largest organ and serves as a primary barrier between us and the outside world. 

Tattoo needles penetrate this fortress by stabbing through the outer layer of the skin, the epidermis, and entering the layer beneath, the dermis. Delivering the ink into the dermis rather than the epidermis is the reason why, despite the estimated 5 billion skin cells we shed a day, our fine line botanicals and minimalist geometrical drawings lie on our forearms and upper backs forever.

The dermis is bustling with immune cells. Any foreign bit that arrives in the dermis, whether it be microbe, toxin, or even just dirt, is sensed by these sentinels. If anything signals danger, immune cells will spring into action, igniting an orchestra of killing, inflammatory molecule release, and damage control that results in the eventual clearance of the threat. 

The ink that we pay to have stabbed in our dermises is a mixture of pigments, usually derived from heavy metals. In combination with the massive amount of injury caused by the repetitive stabbing of tattoo needles, these foreign materials set our immune cells off. Their efforts allow us to successfully heal the wounds we get from this trauma. But, like a battle scar, the ink remains. 

The reason why has to do with macrophages, the “big eaters” of the immune system. These large bodyguards exist throughout our body, gobbling up dead and infected cells, as well as foreign molecules. Once a dangerous bit is ingested, it traffics to phagolysosomes, sacs within the macrophages that are stocked with potent enzymes and acids. These enzymes chew the threat up into soluble waste materials. 

It turns out that tattoo ink is invulnerable to this digestive process. Molecules of ink remain inside the macrophages, staining these immune cell fighters “True Black” or “Sailor Jerry Red” (yes, these are real colors). Eventually these ink-laden cells resign from duty and die in place, releasing the pigmented molecules. Until relatively recently, scientists were perplexed as to why tattoos remain beyond the lifespan of these macrophages. 

But recent work has supported a model in which young, wannabe macrophages in the blood, known as monocytes, quickly infiltrate the dermis, eat up the released pigments, and sit pretty in “Prince Albert Pink” or “Bahama Blue” (also real). And over time, the cycle repeats, with each round of young macrophages eager to chomp up the pigments.

Macrophages have another cool way of keeping those sexy ladies perfectly pinned up on biceps. During the tattooing process, the epidermis is inundated with pigment molecules, the volume and size of which are unable to be completely vacuumed up. At some point, macrophages realize they can’t completely eliminate the ink and instead focus on sequestering it. With the help of some other cells, macrophages form a wall of themselves to imprison the larger ink molecules. 

We’ve capitalized on our skin macrophages’ unending loyalty to the dermis to get life-long works of body art. But though our tattoos last forever, over time they start to lose their pop. The lines fade and the ink colors dim, turning a razor-thin, bright orange tiger into a fuzzy, yellow cat. This is likely due to the dispersion of some of the pigment particles in between cycles of stained macrophages dying and new cells coming to clean up the mess. 

Because macrophages are key to what makes a tattoo permanent, they also lie at the crux of their elimination. Laser tattoo removal is the most common method for getting rid of unwanted body art. During this process, specialized short pulse lasers blast the large chunks of ink into tinier bits that can be taken up and cleared away by smaller immune cells. 

Repeated series of laser therapy can turn your “Amanda” into an “Amanda” and then “Amanda,” until your chest is a finally clean canvas again, all ready for “Michelle.” But the continual cycle of macrophage ink-eating and dying can make it tricky for these therapies to completely remove all traces of a tattoo. Scientists have posited combining laser therapies with localized macrophage depletion. 

I feel kind of sorry for my macrophages, trying their darndest and failing to removal all this ink I let my cousin stick in my precious dermis. But I also think it’s pretty cool. The winding lines of my lizard, meant to represent mountains and hills, are perfect paths of macrophage graveyards, testaments to the persistence and tenacity of my loyal defenders.