How does one become sensitized to nickel? As Jacob explains it, for some people the intimate contact between the skin and the nickel used in earrings and ear posts can do the trick.
"In general, the first time you're in contact with nickel, you don't get a rash," said Jacob. But your immune system remembers the metal and eventually, after more exposure, you reach an "elicitation threshold" when "your skin is primed to react."
Jacob says she wants the American Academy of Dermatology to help push for regulations to limit the amount of nickel in products with prolonged skin contact, just as was done in Europe in the early '90s.
For thousands of years, people in North Africa, the Middle East and South Asia have decorated their bodies with henna, but in recent decades, adulterations in the dye used for these temporary tattoos have resulted in painful allergic reactions.
Henna is a normally reddish-brown or greenish-brown vegetable coloring derived from lawsone, an ingredient found in the leaves of the henna shrub. Normal henna rarely causes allergic contact dermatitis, but darker henna containing an added chemical also found in hair dye called p-phenylenediamine, or PPD, can instigate severe skin problems and additional allergies.
In some cases, the tattooed skin of those who are allergic to PPD will swell and blister, with the contours of the bumpy skin conforming to the shape of the original tattoo.
"When they add the black hair dye at significant concentrations … we're seeing people with blistering and scarring to it," said Jacob.
It's not unusual for people who react to black henna to also react to hair dye, sometimes as a result of the black henna encounter.
While PPD in small amounts is still allowed in hair dyes, it has been banned from skin products in the United States since 1938. Still, henna artists hoping to capitalize on the popularity of the black coloring can mix the dangerous paste on their own.
"When you see somebody with a stand that says 'henna,' ask them what color the stain is when the paste comes off the skin," explained Catherine Cartwright-Jones, a retired henna artist and webmaster for the Henna Page. If the stain on your skin is black, "you've got a problem."
When we think of food allergies, we typically envision a reaction that results after the allergic person puts the problematic food in his or her mouth. But that's not always how it goes.
Dr. Clifford Bassett, an allergy specialist at the Long Island College Hospital and the American Academy of Allergy, Asthma and Immunology, notes that 40 percent of food allergy reactions come not from eating a food trigger but from touching or inhaling it.
Such was the case with one of Bassett's patients, a young girl who developed hives after she hugged and kissed her father when he returned home from work.
"My job is to be sort of an investigative reporter," explained Bassett. "One of the things that we look at is contacts, things we put on the body."
In this case, it wasn't even something the daughter had put on herself. It turned out that the child was allergic to the nut-derived oil in her father's shaving cream, said Bassett.
"That was an episode of a food allergen present in a cosmetic, basically," said Bassett, who added that the problem went away after dad switched to another type of shaving cream.