The Curious Link Between Allergies and Cancer

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Certain environmental exposures are known to increase your risk of cancer: tobacco smoke, UV radiation and even pollution. But what about some common metals and chemicals that cause contact allergies?

Curiously, according to the latest research from scientists in Denmark, these environmental allergens may actually help protect some people from developing cancer. After studying more than 17,000 adults who were tested for common contact allergies — for instance, to nickel and other chemicals — lead author Kaare Engkilde of the National Allergy Research Center at Copenhagen University Hospital Gentofte concluded that those who tested positive for such allergies were less likely to develop cancer years later than those who did not have contact allergies.

Engkilde’s team matched up the allergy test results, which were collected in a database from 1984 to 2008, with Denmark’s national cancer registry, which has recorded all cases of diagnosed cancer among citizens since 1943. For some of the participants, therefore, the study follow-up stretched to nearly three decades.

About a third of people tested positive for contact allergies, which means they developed a rash or swelling when researchers exposed a patch of their skin to an allergen. A smaller proportion, nearly 20% of all of the people tested, went on to develop some type of growth, either malignant or benign; of these, about 38% had tested positive for allergy.

The researchers found that having a contact allergy was associated with a strong protective effect against cancer: those who tested positive for allergies were less likely to develop cancer in later years than those who didn’t have allergies. The relationship was strongest for breast and skin cancers that weren’t due to melanoma.

What do allergies have to do with cancer? It’s not such a far-fetched connection, says Engkilde. A growing body of evidence suggests that our bodies may have natural defenses against cancer, attacking the first signs of tumors much the way we target invading viruses or bacteria. Since allergic reactions are essentially heightened immune responses to often benign compounds, such as dust or nickel or other agents, people with allergies might already be primed for any foreign intruders, including tumors. “People with allergies seem to have less cancer or have fewer different cancer types than patients who don’t have allergies,” says Engkilde. “The reason for this is uncertain but it might have to do with the immune surveillance theory, which speculates that patients with allergies may have a more ready and observant immune system that could lead to earlier detection of cancerous cells.”

It’s important to understand exactly how far the study goes in connecting contact allergies to cancer, and where it stops short in shoring up the link. While the finding is worthy of further study, Engkilde says it’s only the first step in better exposing how the immune system might be influencing the cancer process. For example, his study found an apparent protective effect of allergy against breast and certain skin cancers, but not other cancers; that effect may be attributable to the fact that these cancers are more common, and therefore there were enough cases to generate a measurable correlation. There may well be a similar inverse relationship between allergies and other types of cancers such as colon and prostate, but there simply weren’t enough cases of these cancers to register a statistically significant connection to allergies.

The study also found that people with contact allergies were more likely to develop cancer of the bladder, compared with non-allergic people. Why that’s so isn’t clear. Perhaps it’s the result of increased exposure to chemicals: previous studies have shown that hairdressers, who tend to have higher-than-normal exposure to potentially harmful chemicals during the workday, also have higher rates of bladder cancer. But whether the bladder cancer link is due to allergen metabolites accumulating in the bladder, or whether bladder tissue is different in some way immunologically isn’t known.

It’s also not clear yet whether there is any other health benefit to the link between allergies and cancer —whether, for example, people with allergies end up living longer because they are better able to control their cancer using their immune systems. Engkilde is hoping to pursue that question in mice. At the very least, the findings offer some support for the idea of harnessing the immune system to fight cancer. And any new strategies against tumors, says Engkilde, are always welcome.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.