Macrophage immune cells in the skin can capture pigments from tattoos and hold them in place [nytimes.com], causing tattoos to persist even after new macrophages move in to deal with the contaminants:
That's what French scientists observed from studying tatted mice. In their model of tattoo persistence [rupress.org] [open, DOI: 10.1084/jem.20171608] [DX [doi.org]], published Tuesday in the Journal of Experimental Medicine, macrophages — immune cells that ingest foreign or unhealthy debris in the body — play a starring role. Targeting these cells, the authors suggested, might help improve tattoo removal procedures for people.
As a tattoo is given, macrophages descend to capture invading ink. Probably because the ink granules are too bulky for the microscopic Pac-Mans to break down, they hold onto the pigment, your body art shining through their bellies.
With time, these original macrophages die and release their pigments, which get vacuumed up by new macrophages, starting the cycle over, said Sandrine Henri, a researcher at the Immunology Center of Marseille-Luminy who led the study with her colleague Bernard Malissen.
[...] Jared Jagdeo, a dermatologist at the University of California, Davis, has also wondered whether macrophages impede tattoo removal by reabsorbing lasered ink particles. Since 2014, he has performed a laser removal procedure that uses anti-inflammatory ointment to suppress macrophages. "It makes a difference," he said, noting that he often removes tattoos in 10 or fewer treatments [instead of 20].