Once the social symbol of sailors and jail-hardened individuals, tattoos have surged in popularity among the global youth, with around 40% of adults between the ages of 18 and 29 sporting some ink. Tattoo artists spend years training and a lifetime honing their craft, often specializing in one of many forms of skin-based expression ranging anywhere from portraits to calligraphy to watercolor. Whether you’re a fan of the “I-Love-Mom” classics or elaborate Monet-esque mosaics, the practice of tattooing seems almost magical in nature – and like many other tattoo-lovers out there, you’ve probably found yourself wondering: how in the world do tattoos even work?

Although an ever-evolving artistic platform, tattooing is also a science based entirely around the human immune system. The immune system serves to protect our bodies from foreign invaders like bacteria, viruses, and parasites, using a battalion of specialized cells and signals to fight off and prevent infections – but those pesky E coli. aren’t the only invaders the immune system watches out for. Any trespasser into the body, whether biological or chemical in nature, will draw the immune system’s fire – including tattoo ink!

The science behind tattooing has been a bit elusive historically, but advances in research along with plain-ole’ scientific curiosity have shed some light on these painful skin-paintings. A new study published earlier this year provides an explanation for how exactly tattoos stay put for years and years, and the microscopic MVP behind the art? Dermal macrophages.

A macrophage is basically the Pac-Man of your immune system: eating all enemies in its path, but instead of colorful ghosts, macrophages deal in bacteria and damaged cells. Macrophages are specialized immune cells that serve to grab foreign substances and trap them through a process called endocytosis. Once a macrophage has engulfed something, it works to destroy the foreign material with enzymes before moving on to the next target. After the skin is injected with tattoo ink, local macrophages swoop in to do their programmed duty and ingest the ink particles. These biological Pac-Men lock the ink within their cell walls and hold it there – and when the macrophage eventually withers and dies, the tattoo ink remains where it was, pigmentation unaffected. Eventually, another macrophage will move in to replace the dead one, and the cycle is repeated indefinitely.

Put simply, your body is waging a constant war against that infinity symbol on your wrist – so help your tattoos win by protecting them with a little sunscreen once in a while; tattoos may be resistant to the immune system, but the sun is a different story.