Well, it tries… and succeeds in its own way. However, success on the part of the immune system may actually contribute to the permanence of a tattoo.
Skin is an incredible organ. It represents approximately 7% of one’s body mass and is responsible for very important tasks such as regulating body temperature, restricting water loss, and protecting all of our other organs from bacteria and chemicals. In order to maximize its ability to ward off invading pathogens and particles, the skin is immunologically robust. In the skin, the immune system primarily employs nonspecific reactions to fend off any particles that are not recognized as “self.” These “non-self” imposters can be anything from bacteria in a cut to a splinter. When the skin’s immune cells (macrophages and phagocytes) recognize one of these non-self threats, there is a rapid immune response that leads to mast cell activation. At this point, skin may be red and warm; the recruitment of additional immune cells can lead to the build-up of puss (dead phagocytes that ingested the non-self threat and then got broken down by macrophages). Continued mast cell activation can lead to fibroblast activation. Fibroblasts make collagen, which can create a network and scar tissue.
The process of tattooing does not go unnoticed by the skin’s immune army. A tattoo consists of many small ink globules being inserted into the skin below the epidermis and partly into the dermis. A tattoo needle rapidly inserts a globule of ink comes out of the skin and then reinserts another globule of ink in a slightly different spot. The rate of perforations per minute will depend on whether the tattoo machine being used is a liner or shader and on how many needles are in the tattoo machine at any one time. The continued assault is undoubtedly seen to the immune system as thousands of invasions to the skin barrier. Because the skin surface is compromised, antibacterial ointments are a must for the newly-tattooed. The immune process described above will ensue…macrophages, phagocytes, mast cells, and then fibroblasts. The challenge for the immune system is that the ink globules are too big for the phagocytes to ingest. The collagen network that is built up by the fibroblasts envelopes the ink globules and holds them in place, thereby preventing the invaders from moving. However, this also contributes to the permanence of the tattoo because the ink is suspended in the sub-epidermal space.
Over time, exposure to sun can break down the ink globules into phagocytic bite-sized pieces. Following UV light exposure, the ink breaks down and is carried off by scavengers of the immune system. This process contributes to the fading of tattoos, although the vigilant use of sun block can protect the tattoo. If a person chooses to get a tattoo removed, a laser is used to break the ink globules into smaller pieces that the immune system can clear away. Some scar tissue may remain.
One interesting line of recent research is the use of tattoos to deliver vaccines. Because the skin’s immune system is extremely responsive to continued perforation with a tattoo needle, the process of tattooing may act as an adjuvant that stimulates the immune system’s recognition of a vaccination. This line of research also examines the fact that the skin’s immune forces can support a specific immune response in the rest of the body. Although this process may be useful for domesticated animals that need to be branded, it is unlikely that babies will be receiving a skull tattoo on their bicep during their postnatal vaccination schedule.
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