system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If our cells are always dying and being replaced how does a tattoo last forever?

I've read that over a period of every 7 years or so that every cell in our body has been replaced by a new cell. If you're dying or painting or doing whatever to skin/dermis cells for a tattoo, how does it remain after the old cells have died?
gpt
The ink is injected in to the dermis layer which contains nerves, blood vessels, lymphatic and various other tissues. This layer is not only much more inner than the epiderms (outermost layer), it's also heavily inundated by immune cells ready to fight foreign infections. When the tattoo ink is injected into the dermis layer, immune cells such as macrophages will immediately want to eat up or phagocytose these droplets because they are foreign in nature. The simple way of understanding it is that these immune cells cannot eat up the ink droplets due to their size in nature and will start to aggregate around the ink droplets. The ink then becomes trapped in these aggregates of immune cells that will almost indefinitely stay in the region. These aggregates will continue to stay in the region despite cellular proliferation around them. Laser removal of tattoo works by shrinking these ink droplets to small enough sizes such that they can be phagocytosed by immune cells and excreted from the system.