After finding several lumps under her arms, an Australian woman went to the doctor suspecting she had cancer.
Doctors determined the lumps were enlarged lymph nodes and performed a body scan that showed there were more enlarged nodes in the woman's chest.
"Ninety-nine times out of 100, this will be lymphoma," Dr. Christian Bryant, one of the woman's doctors in Sydney, told CNN.
After surgically removing a lymph node from the woman's armpit, doctors were shocked to find the node was inflamed by black pigment from a 15-year-old back tattoo, not cancer.
Doctors describe the 30-year-old woman's case in the medical journal Annals of Internal Medicine.
"We concluded that the diagnosis was granulomatous lymphadenitis, which was probably a hypersensitivity reaction to tattoo pigment," doctors wrote in the article, according to The Independent.
The tattoo pigment likely made its way to the woman's lymph nodes via the skin's immune cells, Dr. Bill Stebbins, director of cosmetic dermatology at Vanderbilt University Medical Center, told CNN.
Those immune cells ingest foreign substances, like tattoo pigment, and eventually wind up in the lymph nodes.
A study done by the European Synchrotron Radiation Facility reflects Stebbins' comments.
"We already knew that pigment from tattoos would travel to the lymph nodes because of visual evidence: the lymph nodes became tinted with the color of the tattoo," Bernhard Hesse, one of the study's authors, told The Telegraph.
The nodes become tinted because "the pigment is too large for these cells to eat and digest," Stebbins said. "That's why they're still there many years later."
Doctors are still baffled as to why it took 15 years for the unnamed woman to suffer a reaction from her tattoo. Bryant told CNN that he and his colleagues had never seen anything like this case before.
"I think there's absolutely no way to know how common it is," Bryant said. "Most people who have tattoos have absolutely no problems."
A 2012 poll found that 21 percent of all American adults have a tattoo, according to NPR. A seperate poll found that 38 percent of millennials are inked.
"It's important for physicians to be aware of a tattoo history," Stebbins told CNN. People may not realize that it can take years for tattoos to cause inflammation and doctors may not notice tattoos that are under clothing or easily concealed.
Sometimes people develop allergic reactions to tattoo ink. But that tends to happen with red ink, not black, according to CNN.
"No one checks the chemical composition of the colors, but our study shows that maybe they should," said Hiram Castillo, one of the ESRF study's authors, according to The Independent.