Illustration showing a type of white blood cell known as a macrophage (purple) releasing tattoo ink (black) which it had engulfed. During tattooing, ink is injected into the second skin layer, known as the dermis, and triggers an immune response. As part of this response, macrophages engulf the ink, in a process known as phagocytosis. Typically, substances engulfed by macrophages are broken down by enzymes within structures called lysosomes (top-left of cell), but ink remains intact inside these cells in a structure known as the vacuole (sphere around ink). When the macrophages die, the ink is released and is re-captured by more macrophages. This cycle is how the ink remains in the skin, and tattoos become permanent.