Why can’t our skin heal away tattoos?

The way our immune cells protect us holds the secret to permanent ink. Macrophages—specialized immune cells—gobble up invaders, like bacteria and viruses, and destroy them using acid. When tattoo ink is placed between your skin cells, it’s also treated like a foreign invader. There’s a problem, though. Tattoo ink cannot be destroyed by acid. So macrophages do the next best thing: contain it until they die. Once they do, another macrophage immediately swallows the ink again. This indefinite process leaves you with body art that lasts a lifetime.