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Peanut allergy has become a nemesis for increasing numbers of children and parents in recent years, forcing them to maintain nut-free households and prompting many schools to ban a childhood staple, peanut butter, from the lunchroom.

When a child is allergic to peanuts, families must closely monitor everything the child eats both in and outside the home, because accidental consumption of peanuts could prove fatal.

The prevalence of peanut allergy among children in the United States has risen more than threefold, to 1.4 percent in 2010 from 0.4 percent in 1997, according to a study by food allergists at Mount Sinai Hospital in New York City. Up to 40 percent of people with an allergy to peanuts are also allergic to one or more tree nuts, like walnuts, pecans or almonds.

The Food Allergen Labeling and Consumer Protection Act requires that packaged foods which contain peanuts (and seven other food allergens) say so on the label. Some, but not all, food manufacturers also state on packaging if their foods were prepared in a facility that also processes nuts, but such labeling is voluntary.

Some cities have nut-free bakeries that now sell products safe for allergic children, who can bring their own special, albeit expensive, cake or cupcake to a party.

While experts doubt the necessity of some extreme measures taken to prevent indirect exposure to peanuts, the danger to someone with a peanut allergy who eats them is unquestioned.

The potentially fatal reaction, called anaphylaxis, can occur with a child’s first exposure to peanuts: itchiness, swelling of the tongue and throat, constriction of the airway, a precipitous drop in blood pressure, rapid heart rate, fainting, nausea and vomiting.

Unless the reaction is stopped by an injection of epinephrine (adrenaline), anaphylaxis can kill. In one infamous instance in 1986, Katherine Brodsky, 18, a freshman at Brown University with a known nut allergy, died after eating chili that a restaurant had thickened with peanut butter.

There is no cure for nut allergies, although several preliminary studies suggest that it may be possible to temper a reaction to peanuts with immunotherapy. Like shots given for pollen allergies, the approach starts with exposure under the tongue to a minuscule amount of the offending peanut protein, followed by exposure to gradually increasing amounts under strict medical supervision.

The latest study, conducted in Cambridge, England, and published in the Lancet last week, found that after six months of oral immunotherapy, up to 91 percent of children ages 7 to 16 could safely ingest about five peanuts a day, far more than they could before the treatment. About one-fifth of treated children reacted to ingested peanuts, but most reactions were mild, usually an itchy mouth. Only one child of the 99 studied had a serious reaction.

When immunotherapy works, the research suggests, the severity of the allergy is lessened, enabling an allergic person to safely ingest small amounts of the offending protein. It is not known how long protection lasts without continued immunotherapy, however, and the researchers warned that no one should try it on his own. Further study is needed before the treatment can be used clinically, probably years from now.

Meanwhile, everyone with a peanut allergy is advised to carry an EpiPen for emergency treatment.

Ideally, allergists would like to prevent the development of peanut allergy in the first place. Experts had thought that one way would be to keep fetuses and breast-fed babies from exposure to peanut protein by restricting consumption by pregnant and nursing women.

Various studies had suggested that early exposure to peanut protein by infants with allergic tendencies could sensitize them and lead to a serious peanut allergy. In 2000, pregnant and nursing women were advised to avoid eating peanuts, especially if allergies ran in the family. And new mothers were told not to give babies peanuts before age 3, when digestive systems are more fully developed.

But this advice did nothing to curb the steady climb in peanut allergies, and it was abandoned in 2008.

Today, the thinking is exactly the opposite. Instead of restricting exposure to peanut protein by unborn or nursing babies, the tiny amounts that may enter the baby’s circulation when a pregnant or nursing woman eats peanuts might actually induce tolerance, not sensitization.

In a recent study of 8,205 children, 140 of whom had allergies to nuts, researchers found that children whose nonallergic mothers had the highest consumption of peanuts or tree nuts, or both, during pregnancy had the lowest risk of developing a nut allergy. The risk was most reduced among the children of mothers who ate nuts five or more times a month.

The researchers, led by Dr. A. Lindsay Frazier of Dana-Farber/Children’s Hospital Cancer Center in Boston, wrote: “Our study supports the hypothesis that early allergen exposure increases the likelihood of tolerance and thereby lowers the risk of childhood food allergy.” They added that their data “support the recent decisions to rescind recommendations that all mothers avoid peanuts/total nuts during pregnancy and breast-feeding.”

The study was supported by Food Allergy Research and Education, a New York-based nonprofit, and published in December in JAMA Pediatrics.

According to an accompanying editorial by Dr. Ruchi Gupta, an associate professor of pediatrics at Northwestern University, “some studies actually showed that avoiding peanuts during pregnancy increased the risk of a child developing peanut sensitization.”

Further support comes from studies of other common food allergens.

In an Israeli study of 13,019 infants, those who were exposed to cow’s milk protein as a breast-milk supplement in the first two weeks of life were less likely to become sensitive to it than infants first given cow’s milk much later.

An Australian study of 2,589 babies found that those first introduced to egg at or near 1 year of age were more likely to develop an allergy to egg protein than those first given egg at 4 to 6 months of age.