Humans have been tattooing themselves for thousands of years, for spiritual, cultural, or personal reasons. Long before we were covering ourselves in ironic stick'n'pokes, butterfly tramp stamps, or full blackout tattoos, humans were marking their bodies with permanent ink.But while history can tell us much about the culture of tattooing, we still don't know much about how the practice affects our health.Introducing tattoo pigment into the skin triggers an immune response, and that ink obviously sticks around – though it doesn't always stay put.Research shows that particles of tattoo ink can enter the lymphatic system and build up in the lymph nodes.Lymph nodes are key hubs of our bodies' immune activity. Full of white blood cells, lymph fluid is filtered through them to clean out any debris it's collected on the way around your body. That includes misbehaving cells (i.e. cancer), bacteria, viruses, and foreign substances such as tattoo ink.Rapper MGK recently found this out the hard way by rushing a massive blackwork piece that covers much of his torso, including the area of the lymph nodes around his armpits and shoulders. He says the process turned his surrounding skin yellow and made him "really sick."Rapper MGK has extensive body tattoos. (Alexander Tamargo/Stringer/Getty Images)Even without traveling through the body, tattoo pigment 'sets off' the immune system. Pigment enters cells in the dermis, the middle layer of skin, and is swiftly recognized as a foreign substance. The body sends its 'army' of immune cells, but the pigment particles are too large for these cells to remove. That's what makes tattoos permanent, but it also sets up the body for a lifelong battle against an impossible-to-remove invader.What helps make tattoos so permanent is that modern inks are made of a complex cocktail of chemicals."They contain pigments that give color, liquid carriers that help distribute the ink, preservatives to prevent microbial growth, and small amounts of impurities," medical microbiologist Manal Mohammed explained in an article for The Conversation. "Many pigments currently in use were originally developed for industrial applications such as car paint, plastics, and printer toner, rather than for injection into human skin."