From minimalist wrist designs to full sleeves, body art has become so common that it barely raises an eyebrow. But while the personal meaning of a tattoo may be obvious, the biological consequences are far less visible. Once tattoo ink enters the body, it does not stay put. Beneath the skin, tattoo pigments interact with the immune system in ways scientists are only just beginning to understand. Tattoos are generally considered safe, but growing scientific evidence suggests tattoo inks are not