Tattoos have been around for thousands of years, and are more popular today than ever. Tattoos done under standard sanitary conditions don't seem all that dangerous. But have you ever once thought about how your body reacts to the trauma of high speed needles and tattoo ink invading your dermis? Your immune system immediately rushes in to protect you from such foreign substances, but while the system works pretty well against biological invasions such as bacteria and viruses, it's a bit stumped by ink, which can't be killed because it was never alive. The macrophages that normally kill bacteria do the best they can, which ends up actually stabilizing the tattoo! Of course, nothing lasts forever, and that includes macrophages and tattoo ink, which will eventually succumb to your body's ever-renewing systems. But that takes so long that you should still consider a tattoo as permanent. This video from Kurzgesagt is only 7:20; the rest is an ad.