Tattoo ink can be contaminated with bacteria or mold that results from diluting the color pigments with non-sterile water. There is no way customers can tell if the ink is contaminated--even if it's sealed and the product label says it's sterile. Pigments are used to give tattoo ink its color, and the FDA says published research shows that some inks contain pigments used in printer toner or car paint. Note that the FDA has not approved any pigments for injection into the skin for cosmetic purposes.
While many people suffer no negative reaction from the tattooing process, others suffer rashes, including redness or bumps, that could indicate an allergic reaction. Because the inks are permanent, the allergic reaction may persist. Still others develop a high fever, indicating what could be an aggressive infection. Other symptoms include shaking, chills and sweats. Treating such infections might require a variety of antibiotics--possibly for months--or even hospitalization and/or surgery. Contact your health care professional if you have any concerns.
Scar tissue may form when you get a tattoo, or you could develop "granulomas," small knots or bumps that may form around material that the body perceives as foreign. If you tend to get keloids, which are scars that grow beyond normal boundaries, you may develop the same kind of reaction to the tattoo.