Two years after outgrowing a peanut allergy, Holly Sweenie finally took a peanut butter and jelly sandwich to school.

"It took me a long time to get up the courage to pack one," said Sweenie's mom, Susan, of Canton, Mass. "I assumed allergies were something she'd always have."

Happily, allergies aren't always forever. But it can be difficult to tell when kids have aged out of an allergy, and if they have, whether eating previously forbidden foods should be part of the treatment.

About 80 percent of children will lose their sensitivity to milk, egg, wheat and soy allergies, said Dr. Robert Wood, director of pediatric allergy and immunology at Johns Hopkins Children's Center.

Though less common, even life-threatening peanut and tree nut allergies can recede; over time, 20 percent of children will lose their sensitivity to peanuts and 10 percent will be able to tolerate tree nuts.

Some children with respiratory allergies such as seasonal hay fever, meanwhile, can improve and even "outgrow" the allergy as they mature, usually around puberty.

Rates on the rise

Still, allergy rates continue to rise - they've doubled since the 1970s. Scientists don't know what causes allergies or why kids can overcome sensitivity to certain foods but not others. Nor do they know why some have no hay fever in preteen years, but develop it when they hit adolescence.

Treatment generally involves avoidance; the hope is that the immune system will gradually forget about the allergen. But paradoxically, "exposure may help someone with allergies become more tolerant of small amounts," said Wood.

For patients with environmental allergies, allergy pills or shots, also called immunotherapy, are used to help with desensitization. It's thought that gradually retrains the immune system to completely disregard or tolerate the allergens.

For food allergies, however, immunotherapy isn't quite ready for prime time. Preliminary data show that giving children with milk allergies increasingly high doses of milk over time may ease or help them overcome the allergy, but researchers warn it should never be tried at home. Feeding peanuts to someone with peanut allergies, for example, can be life-threatening.

If a child has outgrown the peanut allergy, it may be a different story. Some research shows reintroducing peanuts after a child can tolerate them may help prevent a recurrence. Wood's work has shown children who outgrew peanut allergies and then ate peanuts in one form at least once a month had a lower risk of recurrence than those who didn't.

Still, Wood recommends that children who do eat concentrated peanut products carry epinephrine injections for at least a year after passing an allergy test.

Holly Sweenie was 2 years old when she was diagnosed with peanut, milk and egg allergies. "The toughest part was the meal preparation and sending her off to school," said Susan Sweenie. But one day Sweenie accidentally gave Holly a baked good containing eggs. When she had no reaction, she was retested; the results showed she had outgrown both the egg and milk allergy. A year later, by the time she was 5, tests showed she had outgrown the peanut allergy.