Tattoos are unhealthy, similar to smoking, but they are not necessarily fatal. However, they can cause various complications, ranging from allergies to skin infections from contaminated water sometimes. They can also affect the immune system and increase the risk of cancer, as they contain heavy metals such as mercury, cadmium, lead and arsenic (1). Moreover, if you have an extensive tattoo, the ink will end up in your lymph nodes, and have some impact on hormones too(2). Some people claim that tattoos boost the immune system, but there is not enough studies to support this claim, and the benefit is insignificant compared to the potential risks (3).
(1) https://blog.uvahealth.com/2020/01/13/are-tattoos-bad/amp/
(2) https://www.sfgate.com/health/article/Tattoo-health-risks-re...
(3) https://theweek.com/health-and-science/1022099/how-do-tattoo...