"The particles in pigments are bulky and difficult for an immune cell’s enzymes to degrade. So when inks get gulped down by immune cells such as skin-dwelling macrophages—which spend their lives devouring pathogens, cellular debris, and other schmutz within just a teeny patch of flesh—it can transform into a microscopic version of gum. The pigment particles lodge themselves inside macrophages’ innards, refusing to be broken down. When ink is visible at the surface of the body, it’s not just interlaced among skin cells—it’s shining out from the bellies of macrophages that can’t digest it."

https://www.theatlantic.com/science/archive/2023/03/tattoo-science-immune-system-effects/673462/