From ScienceAlert:
Humans have been tattooing themselves for thousands of years, for spiritual, cultural, or personal reasons. 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.