If you think you aren't allergic to anything, try rubbing poison ivy on your bare skin.
While 99 percent of people will suffer the typical rash and itch, a few will escape the encounter with the natural botanical product unscathed, because they aren't allergic to it.
But poison ivy is not like most other allergens. People who suffer from other skin allergies will also develop a rash from contact with the allergen, but they don't have the vast majority of people who share in their discomfort. And those substances can range from metals such as nickel and chromium, to chemicals such as formaldehyde that can be found in everyday products.
A variety of substances can be allergy culprits, and these allergens can affect you even if you don't inhale or eat them.
Dr. Clifford Bassett, the chair of public education for the American Academy of Allergy, Asthma and Immunology, recalled one young boy who had hives, the source of which turned out to be a his father's moisturizer and shaving cream, which contained nuts. It aggravated the boy's nut allergy when the two embraced.
"It's not just what you put in your mouth, but it may also be contact," he said, noting that allergens can be found in products one would never expect. "You need to be a label detective."
Of course, not all skin itches are due to allergies.
For example, some people think they are allergic to Ivory soap, which is supposed to be moisturizing but can give some a rash. That is likely dry skin, not an allergy, said Dr. Sharon Jacob, assistant professor of medicine and pediatrics at the University of California at San Diego.
"It actually tends to be fairly drying for the skin," she said, noting that she recommends that patients with eczema and dermatitis avoid that brand of soap.
"Like we might say with a medication, it's more of a side effect," said Jacob.
While allergists test patients for foods they eat and allergens they may inhale, dermatologists are often the ones who look at contact allergies.
As opposed to the testing often done by allergists, which involves pricking patients' skin, dermatologists typically conduct a patch test, in which a patch containing the allergen is stuck to the skin for 48 hours and checked again after 72 hours to test for contact allergies.
"We're not testing for foods and those types of things," explained Dr. Bryan E. Anderson, an associate professor of dermatology at Penn State Milton S. Hershey Medical Center. He said this separate test can confuse patients who think they have been already tested for all of their allergies.
While only a dermatologist can tell you if you have a contact allergy, see if you should ask about one of these nine culprits.
Women who regularly use eye makeup may find that it can suddenly cease to agree with their eyelids.
"Very commonly they may use the same makeup for years, and then something changes and now they're getting an itchy, flaky red rash, usually on their eyelids," said Dr. Andy Nish, president of Allergy and Asthma Care Center in Gainesville, Ga.
Over time, the effect of the eye makeup can lessen or get worse, which can make it a hard allergy to detect.
"Sometimes, even though they have the allergy, it sort of cycles," said Nish. "It's not always obvious."