Most people with allergies first develop them as children or infants. But as they age, some individuals seem to leave their hay fever, pet allergies or even food allergies behind.

Doctors don't know exactly why, but people's allergies actually can disappear over time. And even when they don't disappear, allergies vary significantly.

The severity of allergic reactions differs widely among people, and even within the same individual, allergic reactions can change in severity from season to season and from allergen to allergen. For example, a neighbor's cat might send you into a sneezing fit, while a different feline could provoke nary a reaction at all.

In general, doctors do know what causes allergies: Your immune system overreacts to a harmless substance. When functioning correctly, your body's defenses attack foreign invaders, like viruses. With allergies, the immune system mistakenly targets pollen, pet dander or certain foods, for example, sending molecules called immunoglobulin E (IgE) antibodies to orchestrate a "defense."

In cases of disappearing allergies, some experts theorize that the person may simply grow accustomed to the allergen, thus reducing the level of immune-system sensitivity.

"Growing accustomed" seems important in allergies to food, particularly nuts. Some doctors have recently emphasized promoting tolerance to the food through low-level exposure that's gradually increased.

Physicians used to think that nut allergies, particularly the severe variety associated with peanuts, always lasted a lifetime. Over the last decade, however, studies have shown that about 20 percent of children with peanut allergies can overcome the sensitivity.

By looking at allergy blood tests, which show IgE levels, doctors can even characterize a child's chances of outgrowing food allergies.

But even when food reactions seem like they've gone away, the trouble's not necessarily banished; symptoms of food allergies can return just as mysteriously as they disappeared. Allergies to pollen, pet dander and other substances can recur, too.

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