Contact allergies 'may be associated with reduced risk of some cancers'
People with contact allergies to common metals and chemicals may be less likely to develop certain types of cancer, according to a study by Danish scientists.
However, the study only looked at retrospective data rather than actively tracking people's habits over time, so further studies are needed to clarify the apparent links between contact allergies and the risk of cancer.
The research was carried out by scientists at the National Allergy Research Centre in Copenhagen, Denmark.
Scientists analysed data on 16,922 Danish adults, all of whom were tested between 1984 and 2008 to see whether they were allergic to any common contact allergens.
Thirty-five per cent of people reacted to at least one allergen, including 41 per cent of women and 26 per cent of men.
The researchers looked to see whether any of these individuals were recorded on the Danish Cancer Registry, which contains data on all instances of cancer in the country.
They found that 19 per cent of participants had developed a growth (cancerous or non-cancerous), and that 38 per cent of these individuals had a contact allergy.
Overall, the researchers found a strong link between contact allergies and cancer, though from this analysis alone, they were unable to say whether contact allergies were the direct cause of the reduced cancer risk.
Breast cancer and non-melanoma skin cancer were both significantly less common among people with contact allergies, and brain cancer was less likely to develop in women with an allergy.
The researchers suggest that the findings may be explained by the so-called 'immunosurveillance hypothesis', which states that people with allergies may be less likely to develop cancer because their immune systems are highly responsive.
In contrast, the researchers observed that people with contact allergies had an elevated risk of bladder cancer - a trend they believe may be due to higher levels of chemical break-down products in the bladder.
The study, published in the journal BMJ Open, cannot be used to draw conclusions about allergies and individual cancer risk - further research is needed.
Dr Caetano Reis e Sousa, a Cancer Research UK immunology expert, said: "This is an interesting study, but it doesn't tell people with allergies anything about their individual cancer risk. Firstly, the researchers only looked at a specific type of allergy, so this work doesn't apply to other common allergies such as hay fever. Secondly, the study only demonstrated a statistical link, not the actual cause of this relationship. So further work needs to be carried out before scientists can give solid reasons for these associations.
"Nevertheless, this work highlights the value of good quality databases to help scientists explore possible causes of cancer. It opens the door for further research looking at the role of the immune system in allergies and cancer."
The researchers said: "More refined analyses, adjusting for social class and smoking, for instance, and studies focusing on specific chemical exposures are required to further our understanding of the role of contact allergies in the development of cancer.
"However, if these relations are aetiological, there are implications for understanding how contact allergy can affect cancer development, and vice versa."