How tattoos stay put thanks to a war with your immune system

Humans have marked their bodies with permanent tattoos for thousands of years to show off their social status, religious beliefs, declarations of love or just an appreciation of art.  For years tattoos were thought to stay in the skin because a dye was injected deep into the dermal layer of the skin staining the skin cells or fibroblasts located there.  Research published this week in the Journal of Experimental Medicine sheds new light on how tattoos stay stationary in the skin and it’s much more of an active process than previously thought.

Contrary to popular belief, tattoo needles don’t actually penetrate very deep into the skin and instead of injecting like a syringe needle they act more like a fountain pen.  The pen-like needle is dipped into an ink well and used to create a superficial puncture in the skin.  The ink from the needle tip is then drawn down through this puncture wound using capillary action until it reaches the deeper dermal layers of the skin.  Capillary action is the same force that draws fluids into a dry paper towel, and pulls water up through the stem of a plant.

To study where tattoo ink goes, the researchers tattooed the tails of mice with a green fluorescent ink and examined sections of the skin under a microscope.  They found that rather than staining skin cells, the ink particles were encased inside white blood cells called macrophages which were nestled in between the skin cells.

Macrophages are cells released by our bodies perceive that it is under attack..  They rush to the targeted site looking for foreign material and are designed to ingest any foreign or cellular debris to help clear it away and protect the body from any further infiltration.

The study showed that the injury caused by the tattoo needle results in a migration of macrophages to the tattoo site where they gobble up the foreign tattoo ink to protect the body from further invasion.  Because the tattoo ink is relatively large in size, it can’t be drained into the lymph nodes through the lymphatic vessels so rather than drain the debris, the ink containing macrophages just settle into the dermis and keep the body protected from the foreign ink by encapsulating it.

Dermal macrophages which have consumed green tattoo ink in the mouse study. Scale Bar 10 µm Source

To test this macrophage theory further, the researchers used mice that had been genetically engineered to have macrophages that were easy to kill off.  They then exposed the tattooed mice to diphtheria which killed off the macrophages in the local area.  They found that there was no visible change in the tattoo even when the original macrophages were destroyed because new macrophages arrived immediately to eat the invasive ink particles and keep it in place.

Understanding how tattoos remain in the skin and the importance of macrophages could help to develop a new way to remove unwanted tattoos.

Currently pulsed lasers are used to blast macrophages full of ink into chunks small enough that the lymphatic system ca drain them away.  This laser removal method is painful and can take several sessions sometimes only resulting in fading of the tattoo rather than complete removal. 

This new research opens up opportunities that could involve temporarily killing off or removing macrophages in the area with the use of antibodies instead of lasers, which is less painful and could result in faster removal.

So the next time you look at a tattoo and think of it as a static piece of art, remember that tattoos are actually maintained by an ever-changing immune system war which involves cycles of gobbling up pigment, cell death and eating pigment all over again.