Illustration showing the skin tissue of an arm being pierced by a tattoo gun (top). Tattoo guns work by piercing the outer skin layer known as the epidermis (pink) with multiple needles and depositing ink beneath it, in the second skin layer known as the dermis. This triggers an immune response involving various white blood cells (red, blue and purple). As part of this response the ink is taken up by white blood cells called macrophages (purple). Macrophages are unable to break the ink down and when they naturally die, the ink is released and taken up by another macrophage. This cycle continues, giving tattoos their permanence.