Permanent tattoo also means permanent immune action of the macrophages
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Have you ever wondered why a permanent tattoo seemed to last forever? A simple assumption would be is that the ink could have seeped so deep into the skin that it would not have any other way out. However, this is not the case. Studies on the biological aspect of permanent tattoo reveal that the macrophages are engaged in an endless scavenging effort to remove the ink as non-self particles from the skin.
Permanent tattoo and the essence of permanence
People with a permanent tattoo could have cited multifarious reasons for getting one — from a brazen expression of love and art to aesthetic resolves. Behind those multifarious reasons, though, is the enticing essence of the “permanence” it hails. There seems to be no other better way to behold something worth everlasting than getting a permanent tattoo on the skin. Thus, despite the potential long-term medical risks involved (e.g. tattoo-related rash, severe itching, and chronic swelling)1 and the possible regrets after getting inked, some people would not waver going under the needle.
The biology of permanent tattoo
What makes a permanent tattoo “permanent”? Why does it last? The ink used in tattoo is composed of colorants (typically, pigments) combined with a carrier. The carrier (e.g. a distilled water or an alcohol) acts as a solvent of the colorants. An alcohol-based carrier has an added advantage of increasing the permeability of the skin to the colorant. Using a mechanized needle, the ink is injected deep down to the skin all the way through the epidermis. The ink reaches the dermal layer just beneath the epidermis by way of a capillary action.2 The ink does not stay in the spaces between the cells. Rather, different cells in the skin, such as keratinocytes (cells in the epidermis) and fibroblasts (cells in the dermis), take in the pigment molecules. Apart from these cells, the macrophages in the dermis gobble them up as well. These macrophages do so as part of their job as large “eating cells”. The ink is considered as a non-self particle and therefore it must be removed from the body. Some macrophages bring with them the pigment to the lymph nodes.3 Others are trapped in the dermis with the fibroblasts for as long as they live. Nevertheless, the keratinocytes that have taken in the pigment are shed to be replaced with the new ones while ink-laden dermal macrophages die eventually. Thus, one may ask: “how does a permanent tattoo stay put despite the periodic shedding of the skin and the eventual death of ink-laden cells such as macrophages?”
Permanent tattoo and cyclic scavenging of the macrophages
The permanent tattoo in the skin can be considered as a wound. Similar to any other wounds, it leads to the activation of inflammatory pathways as part of the healing process. During the inflammatory phase, the macrophages move to the injured site to scavenge for foreign particles and cellular debris. When the ink for a permanent tattoo is injected into the skin the macrophages gobble them up. Thus, even if the keratinocytes in the epidermis are replaced with new ones a large chunk of the ink remains because some of the macrophages that engulfed the ink stay there for good. These ink-laden macrophages, though, would also eventually die and wither. When this happens, new macrophages are presumed to arrive to the site to engulf withered old macrophages as well as the released ink particles.4
The engulfing, releasing, and re-engulfing of pigments from a permanent tattoo is an indication how macrophages remain loyal to their role. Perhaps, a deeper understanding in this regard could lead to improved tattoo removal methods. But for those who are keeping their permanent tattoo as it is, knowing that the macrophages in incessant immune action as the reason behind its permanence can be quite fascinating.
— written by Maria Victoria Gonzaga
References
1 NYU Langone Medical Center. (2015, May 27). Tattoos may come with long-term medical risks, physicians warn. ScienceDaily. Retrieved from www.sciencedaily.com/releases/2015/05/150527213601.htm
2 Feltman, R. (2018). Tattoos are permanent, but the science behind them just shifted. Popsci.com. Retrieved from https://www.popsci.com/how-tattoos-work
3 European Synchrotron Radiation Facility. (2017). Nanoparticles from tattoos travel inside the body, scientists find. ScienceDaily. Retrieved from www.sciencedaily.com/releases/2017/09/170912093105.htm
4 Baranska, A., Shawket, A., Jouve, M., Baratin, M., Malosse, C., Voluzan, O., Vu Manh, T.P., Fiore, F., Bajénoff, M., Benaroch, P., Dalod, M., Malissen, M., Henri, S., & Malissen, B. (2018). Unveiling skin macrophage dynamics explains both tattoo persistence and strenuous removal. The Journal of Experimental Medicine. DOI: 10.1084/jem.20171608
