In my first year of graduate school, Professor Sam Behar was giving us a lecture about phagocytes, a group of cells that includes macrophages, neutrophils, and a number of other immune cells that tend to gobble things up. These cells are all over the place, and some can stay in the same place for many years. "How long?" he asked, and then clicked to a slide that had a picture of my back.
To be fair, he didn't know it was my back - but all of my classmates did, and it was kind of awkward. The point he was trying to make is that phagocytes take up tattoo ink, but aren't able to break it down. So it sticks around as long as the cells do. And when those cells die, they are generally eaten by a neighboring phagocyte, so the ink doesn't go very far.
My dad objected to my first tattoo because he's from a generation where "only low-lifes and sailors get tattoos," but nowadays, that list needs to at least include scientists too. For a number of years, Carl Zimmer has been curating a collection of science tattoos, and now he's written a book, Science Ink, that will highlight some of his favorites, tell their stories and illuminate the science behind them. It seems like an awesome project, and I'm honored that my back made the cut.