Our bodies are pretty good at ridding themselves of unwanted invaders, so why is it that we are able to inject ink into them, and have it stay? Over time, molecules are broken down by white blood cells and transported away. This does occur with dye molecules (that's part of why new tattoos fade quickly at first), but most of the dye molecules are too big to be picked up by white blood cells. Unable to get rid of them, the body sequesters the dyes into fibroblast cells, where it hangs out basically indefinitely. Over time, however, fibroblasts do die and are replaced, leading to more tattoo fading. The other main reason tattoos fade is due to UV rays from the sun breaking down dye molecules, such that they are small enough for white blood cells to dispose of. It’s in this same manner that laser tattoo removal works- applying light and heat in order to break tattoos down.