“My sister used to sing about how much she loved peanut butter and it made me so sad not to have any,” Blumenfeld said. “Right after we heard from the doctor, I made my mom stop at the grocery store on the way home so I could get some. It was amazing.”

Food allergies in children have risen by nearly 20 percent over the past decade and now affect about 5 percent of all kids, according to the federal Centers for Disease Control and Prevention.

While overtesting and overdiagnosis may be contributing to the rise, researchers agree that environmental factors — such as certain pollutants or reduced exposure to germs — are probably also to blame. In addition to developing better tests, researchers have turned their attention to finding a way to reverse allergies to prevent some of the 9,500 hospitalizations and dozens of deaths that occur in children every year because of severe allergic reactions to food.

Food allergies occur when the immune system incorrectly identifies certain proteins in foods as harmful, triggering a cascade of reactions that lead to symptoms and, rarely, life-threatening anaphylactic shock.

In a small study published two months ago in the New England Journal of Medicine , 28 percent of children who were treated for their egg allergies by consuming increasing doses of egg protein under a physician’s supervision for nearly two years were able to overcome their allergy.

While it’s a promising start, “we’d like to get better results,” said study coauthor Dr. Wayne Shreffler, director of the food allergy center at Massachusetts General Hospital. More than two-thirds of the children weren’t cured. And 4 of the 40 children who were treated withdrew from the study after experiencing mild allergic reactions such as hives or itchy throat. None had severe reactions, but the study didn’t include anyone who had life-threatening allergic reactions to eggs in the past.

“We still have a lot of unanswered questions about the safety of this technique,” said study coauthor Dr. Marshall Plaut, an allergy research administrator at the National Institute of Allergy and Infectious Diseases. But researchers are pressing ahead, with federal funding, to study the technique for treating peanut allergies, which are more frequently associated with serious allergic reactions. Only 20 percent of children outgrow peanut allergies — compared with 80 percent who outgrow other food allergies.

Shreffler has several new trials underway to test variations on this technique — called desensitization — using both increasing doses of food and an experimental skin patch swabbed with peanut protein. The second method is needed in part because many children learn to hate the smell and taste of foods that trigger hives, throat itchiness, breathing difficulties, or vomiting, so getting them to eat these foods in studies has proven to be a challenge, Shreffler said.

Even as they search for a cure, allergy experts don’t completely understand how and why food allergies occur.

The latest research suggests that the American Academy of Pediatrics’ efforts to curb food allergies during the 1990s — by issuing a recommendation for parents to delay feeding babies with allergy-prone siblings allergenic foods such as nuts, eggs, and fish for the first few years of life — may have backfired badly, contributing to a greater number of children with allergies. In a 2008 study, British researchers found that Israeli babies whose early diet included a soft peanut-based snack were 10 times less likely to develop food allergies in elementary school than their counterparts raised in Britain who weren’t exposed to peanuts as babies.