How do tattoos stay permanent?

The outer layer of skin, the epidermis, constantly sheds and renews itself, so ink placed there would fade within weeks. A tattoo machine punctures the skin between about 50 and 3,000 times a minute, delivering ink roughly 1 to 2 millimetres deep into the more stable dermis. The body treats the ink as a foreign invader, and immune cells called macrophages swallow the pigment particles. A 2018 study in mice found that when these cells die, the ink is released and quickly taken up by new macrophages, keeping the tattoo in the same spot. Laser removal works by shattering the pigment into smaller pieces that the body can gradually clear away.