A woman in Australia was diagnosed with lymphoma at first, but doctors were startled to find black tattoo ink in her swollen lymph node that was from 15 years prior and to which her immune system was only now reacting.

They discussed the odd circumstance in an article that was published on Monday in the journal Annals of Internal Medicine.

The 30-year-old woman arrived to the clinic with little lumps under her arms that had been there for two weeks. More enlarged lymph nodes were discovered in her chest during a body scan, particularly near the roots of her lungs.

Hematologist Dr. Christian Bryant of Sydney’s Royal Prince Alfred Hospital asserted that lymphoma would almost always be detected. Bryant is one of the woman’s doctors.

Lymph nodes can enlarge due to a variety of cancers, including lymphomas, which arise from immune cells. Expanding lymph nodes can also be brought on by infection and inflammation.

The Australian woman, whose name has been protected, had swollen lymph nodes due to an allergic reaction to the previous tattoo ink rather than cancer cells. The medical team removed her armpit lymph node and found a clump of immune cells that were strongly black colored. The woman had a large, older tattoo on her shoulder and a smaller, more recent one on her back.

The skin contains its own immune cells that are constantly monitoring the skin, according to Dr. Bill Stebbins, director of cosmetic dermatology at Vanderbilt University Medical Center.

These immune cells absorbed the tattoo pigment after identifying it as a foreign substance, and over time they traveled from the skin to the lymph nodes.

The pigment is “too large for these cells to swallow and digest,” claims Stebbins. That explains why they have remained there for so long.

But 15 years after having her tattoo, it’s unknown why the woman responded in that way. According to Bryant, something caused the immune cells to become active, but her doctors were unable to determine what it was. The patient said that her tattoos occasionally itched, even though it only lasted a few days each month. The inflammatory response known as a granuloma that was present in her lymph nodes was absent from her skin.

Bryant and his colleagues had never seen anything like it. Despite numerous reports reporting swollen, pigmented lymph nodes that were mistaken for melanoma, this is the first occurrence with lymph nodes deep enough to fit the description of lymphoma that they have heard of.

There was “just no way to tell how common” it was, according to Bryant. “The vast majority of tattoo owners are completely trouble-free.”

“We do a lot of tattoo removal with lasers in our practice, and sometimes we have people experiencing allergic reactions to the ink,” said Dr. Bruce Katz, the director of the Juva Skin and Laser Center in Manhattan and a fellow of the American Academy of Dermatology.

However, rather than black pigments, similar reactions usually happen to red pigments.

Over four out of ten millennials in the US, according to a Pew Research Center research, have tattoos. The percentage of Generation Xers with tattoos is just under 32%. For baby boomers, that proportion drops to 15%.

To prevent infections, Katz encouraged anybody thinking about getting tattoos to do their homework: check the reputation of the tattoo artist, ask for client references, and make sure they’re using disposable needles and unopened ink.

According to Stebbins, people should be aware of these rare issues because they might not be aware that a tattoo can result in inflammation years down the road. Doctors should be informed about a patient’s tattoo history, he continued.

Tattoos are typically hidden from view, so a doctor might not see them right away. Nearly three-quarters of Americans with tattoos often cover them with clothing, according to a Pew survey.

Additional negative effects from getting a tattoo could include infections and other inflammatory reactions. A Texas man died despite swimming with a recently tattooed body part that was infected with a disease common to coastal waters.

However, the Australian woman’s situation is considerably less serious. Since Bryant last saw her, the swollen lymph nodes had subsided.