If you were diagnosed with an allergy to peanuts as a child, you have no doubt lived your adult life in dread of a stray nut, or its oil, finding its way into your food. Only one one-thousandth of a peanut can send susceptible people into full-blown anaphylactic shock: their blood pressure plummets, their breathing becomes labored, and their tongues and throats can swell to the point of suffocation. Indeed, peanut allergiesthe most severe of all food allergieskill an estimated 100 people each year. Given the threat, few would even think of being retested.

"Until now, the rules have been that when you diagnose a patient with peanut allergy, tell them that it is a life-long allergy," says Robert Wood of Johns Hopkins University. In a new study, however, Wood and his colleagues discovered that roughly 20 percent of 223 patients with well-documented peanut allergies no longer had adverse reactions. "I recommend from this study that children with peanut allergy be retested on a regular basis, every one or two years," Wood adds. Adults who have not had reactions since childhood should similarly undergo reevaluation, he says.

To conduct the study safely, Wood and his colleagues first gave skin tests to all 223 subjects. They also measured the amount of Immunoglobulin E (IgE)an antibody that causes allergic reactionsin their blood. Only those patients who had sufficiently low IgE levels were invited to ingest a four-gram dose of peanut protein. Of the 85 eligible and willing participants, 48 suffered no consequences. "People who may have outgrown their allergy, based on the criteria established in this study, should definitely go through a formal oral challenge under a doctor's supervision," Wood says. "Relieving the burden of fear that is caused by peanut allergy is easily worth going through the challenge."