Olga Khazan, writing for The Atlantic:
It turns out your ethereal watercolored bird is kind of like an infection—and the reason it's permanent is because your body keeps on fighting it forever. Tattoo needles punch through the epidermis, the outer layer of skin, and drive the ink into the dermis, the deeper layer that's mottled with nerves and blood vessels. "Every time the needle penetrates, it causes a wound that alerts the body to begin the inflammatory process," the video explains. That signal sends immune system cells racing to the site of the wound (or multiple wounds, in the case of the five-inch dragon breathing fire across your chest). Special cells called macrophages come to the rescue, eating up the dye in an attempt to "clean up" the inflammation it's causing. The rest of the dye gets soaked up by skin cells called fibroblasts. The fibroblasts, along with many of the macrophages, stay suspended in the dermis in perpetuity. The dye in the bellies of the trapped macrophages and fibroblasts shows through the skin, projecting your Chinese word for "love" or constellation of tiny blue stars to the outside world. I have a couple tattoos, so this was interesting to learn. Be sure to watch the video.