The Biology of Tattoos
Tattooing has been a cultural practice for thousands of years, dating as far back as 3300BCE. Since the earliest applications of the practice, technology has evolved to make this popular form of body adornment more precise, diverse, and permanent. Modern tattooing generally involves an automated tattoo “gun,” which allows for the efficient depositing of ink into the dermis through a series of repeated punctures with ink-soaked needles. Tattoos are, of course, permanent, though the mechanism behind how the ink stays in the skin for decades is more nuanced than one might initially think.
The permanence of tattoos can seem counterintuitive considering the body’s natural response to the presence foreign bodies; however, it is this very immune response that allows for tattoo ink to remain in the skin permanently. Tattoo needles puncture the skin repeatedly, reaching the second layer of skin (dermis), where ink is deposited. This layer of skin is rich in blood vessels, nerves, and immune cells such as T cells and macrophages. Upon injury, immune cells flood the area and cause swelling, partially carrying away damaged cells and nanoparticles of ink. Only a tiny amount of the ink is actually removed from the initial site of the tattoo, while the rest remains in the dermis.
In a 2018 study carried out by Baranska et al., it was found that the macrophages that are sent by the immune system to attack the foreign tattoo ink are actually crucial to its survival within the skin. A release-recapture cycle involves macrophages phagocytosing (engulfing) the ink, which is then trapped in a storage sac called the vacuole. Within the vacuole, the ink remains surprisingly unharmed, which is not the case for most foreign bodies engulfed by macrophages. When the macrophage naturally dies, the ink is released from the vacuole, only to be recaptured by the next macrophage. This study directly disproved formerly accepted ideas as to how tattoos remained in the skin, which hypothesized that either ink molecules were simply too large to be taken up by macrophages or that the ink directly stained fibroblast cells. Essentially, the immune response to the tattoo is never ending; the ink simply goes through endless cycles of release and recapture by a progression of macrophages.
It is precisely this process that makes tattoo removal difficult. Although the lasers used in most removal techniques are able to partially break down the ink and kill the macrophages storing the ink particles, much of the released ink is then recaptured by nearby macrophages. Because of this, it often takes multiple laser sessions to significantly fade a tattoo, and total removal is sometimes impossible. New research has shown that by taking advantage of the developing knowledge of the macrophage life cycle, it may be possible to develop removal techniques that target the macrophages themselves, which could potentially lead to a less time-consuming and more cost-effective way to remove tattoos. This method would combine traditional laser therapy with cutting-edge removal of macrophages from the dermis to ensure that the released pigment is not taken up by neighboring macrophages.
Tattoos, although permanent, are inherently dynamic in nature. Understanding the biology behind the permanence of this popular form of body modification is not only interesting for those who choose to get tattooed, but also critical in the development of new technologies for the removal (and perhaps even improvement) of tattoos and tattooing.
References:
https://www.popsci.com/how-tattoos-work/
https://www.popsci.com/science/article/2013-06/fyi-what-makes-tattoos-permanent/
https://www.bio-rad-antibodies.com/blog/how-macrophages-make-tattoos-last.html
https://rupress.org/jem/article/215/4/1115/42419/Unveiling-skin-macrophage-dynamics-explains-both