If you have a contact allergy, merely touching the wrong metal or chemical is enough to send your immune system into overdrive, triggering hives, swelling and general unpleasantness. However, it seems that this sensitivity may have a positive side effect: your hair-trigger immune system could be better at fighting off cancer.
In a long-term study of 17,000 Danish people, scientists found that having contact allergies was correlated with lower occurrences of certain cancers. When adjusted for age and sex, there was a significant negative association between the allergic reaction and breast cancer and non-melanoma skin cancer in both men and women, and brain cancer in women. However, there was an increased risk for bladder cancer.
The researchers think that the reason why having contact allergies could help is what's known as the "immunosurveillance hypothesis", where your immune system so keyed up that it will attack almost anything, making it prepared for cancer, but also freaking out at the drop of the hat. Why the increase in risk for bladder cancer? Possibly, say researchers, it's because allergies and chronic inflammation stimulate cell growth, which gives malignancies a better chance to spread.