Tattoos are injected into the second layer of our skin, the dermis, which rests underneath the protective layer of the epidermis. This alone isn’t enough to make tattoos permanent however, as the damage caused by the needle brings our immune response, including our foreign body-eating white blood cells, to the site. Unfortunately for the white blood cells, the injected pigment molecules are too large to be consumed, meaning the ink can hang around indefinitely.
The permanency of tattoos is a double-edged sword, as they can be difficult to remove. The most effective current approach is laser treatment, which involves targeting light on a single colour. This breaks the pigment molecules into smaller pieces that can subsequently be feasted on by white blood cells.