Illustration showing the immune response to tattoo ink (black). During tattooing, ink is injected into the second skin layer, known as the dermis, and triggers an immune response. Certain types of white blood cell including dendritic cells (blue) and t helper cells (red) chemically signal other immune cells to respond. Macrophages (translucent, bottom-left), another type of white blood cell, engulf the ink. Typically, substances engulfed by macrophages are broken down, but ink remains intact inside these cells. 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.