or Carl and Amy Nathan, milk is the enemy. They have felt that way since their son was 2 1/2 years old, when they saw him physically disintegrate in a matter of minutes after someone at a party fed him something that contained yogurt. Carl and Amy already knew Eric was somewhat allergic, because they had previously seen him respond to milk with wheezing and itching. Still, they weren't prepared for this: in the driveway, on the way to the car, he staggered, retching violently, and his face grew swollen. By the time they were in the car and on their way to the emergency room, they could tell from his gasping that his throat was closing up. "You'd know that sound if you heard it," says Carl, a soft-spoken, precise man who then demonstrates: out of his mouth comes a noise like a sob, rasping and strained. Although he is a doctor, a research immunologist, in fact, he says he felt an unprecedented panic as he reached into the glove compartment for the syringe of adrenaline that was there in case of such a reaction. "There probably isn't a part of the anatomy that I haven't injected a needle into," he says. "But that's very different from being a parent, in the back seat of a car, in the dark, with your son dying in your arms."

The shot relieved Eric's symptoms, but over the next several months, his parents observed that his sensitivity had grown more severe. Once, after she'd had a sip of milk, his mother kissed him good night and saw a lip-shaped welt rise up on his cheek. "Merely touching a table surface that had the taint of milk could provoke an itchy rash," his father explains. "His eyes watered and itched if he walked by a pizzeria." Over the years, tests revealed a host of other, less severe allergies, to corn, soy, eggs and peanuts.

Amy and Carl began reconfiguring the contours of their lives to protect Eric from those allergens. They kept him away from any public place where milk might be served: cafeterias, restaurants, food festivals, fast-food joints. They stopped buying food with milk in it, even trace elements. Amy made arrangements for him to eat his specially prepared lunches in the school library or the band room, on a different floor from the cafeteria. On the days the cafeteria below his classroom served pizza for lunch, she picked him up and whisked him out of school for that hour, lest any milky particles drift upward through the vents.

As he got older, Eric's younger brother, Noah, who is allergy free, started to complain that he felt deprived of too many foods. To placate him, Carl would occasionally take him to a park where, as if engaging in something deviant, he would give him one small container of cottage cheese and a plastic spoon -- and anxiously watch as his son ate it, reminding him not to spill even a little bit on his clothes or shoes. Then they would both scrub their hands and face and return home, at which point Amy would be waiting, change of clothes in hand, to replace any contaminated item of clothing.

Carl Nathan worries about the isolation that Eric, now 17, suffered as a result of his parents' protective measures. Play dates at friends' homes were rare, because few parents seemed to grasp the severity of the problem. "Everyone's well-meaning, but the question is, Who can you really trust?" Carl says. "And the answer turns out to be no one."

s extreme as it would once have sounded, the Nathan family's story -- the early scare, followed by the deployment of elaborate defensive systems and obsessive attention to food -- is increasingly familiar to parents with school-age children. With mounting frequency, school-board meetings and class orientations cover the subject of children with hair-trigger allergies: the girl who could go into anaphylactic shock, a multi-organ allergic reaction, if she so much as touches a piece of cheese, or the boy who can't breathe if children around him are eating peanut butter. No one has been tracking the numbers until recently, but school principals, summer-camp owners and most pediatric allergists agree: potentially life-threatening allergies -- most often to peanuts, technically a legume, but also to nuts in general, milk, egg, soy, wheat, corn, fish and shellfish -- are on the rise in this country. It is estimated that 5 to 8 percent of children under 3, and up to 3 percent of school-age children, have true food allergies. Among adults, the number is smaller -- about 2 percent -- in part because many people outgrow their allergies and in part because whatever causes these allergies in the first place is growing steadily more common with each new generation.

The culture of food has shifted in response, in profound ways that might nevertheless go unnoticed by anyone who wasn't looking for them. In recent years, the ingredients lists on packaged foods have grown so meticulous that they can seem more like hazardous-material warnings. At the end of May, responding to Congressional pressure, America's two largest food-industry groups issued voluntary guidelines calling for the additional listing of trace ingredients -- previously lumped under the somewhat dubious term "natural flavors." General Mills has installed doors in all its equipment so that it can more easily be inspected for stray ingredients; in Hershey's plants, some manufacturing lines are reserved for nut products and nut products alone, to avoid cross-contamination of other lines.

"There's been a revolution among major manufacturers in the past five years," says Susan Hefle, co-director of the food-allergy research and resource program at the University of Nebraska. "You see more and more companies blowing the whistles on themselves because they're afraid of making someone sick. You never saw that before."

But the most visible accommodations have been made where children encounter food -- schools, summer camps, child-care centers. Under the Americans with Disabilities Act, these institutions are required to make adequate provisions for children with allergies, although how they do so is variable. Some camps have banned nuts altogether. Schoolchildren are increasingly accustomed to being segregated by diet -- those who want to eat peanut butter and jelly sit at this table; those who absolutely cannot sit across the room. As a result, certain rituals of childhood that had long been taken for granted, like sandwich swapping at lunch, are now frequently off limits, with staff members enforcing barter bans. Table-wiping procedures are a matter for group committees. And learning about their classmates' food allergies is becoming a topic of general safety for children, along with looking both ways before crossing. As of this August, the Girl Scouts of America will introduce a special merit badge for girls who have learned how to help a food-allergic friend.

Nonetheless it remains, more often than not, the burden of the individual parent to find a solution and then persuade the school of its necessity. When her daughter Jaila was in kindergarten, Cathy DeRienzo found herself heading down to the school as often as once a week, responding to a call from the nurse's office that Jaila, who was allergic to milk and eggs, was ill again. DeRienzo knew what was triggering the hives, what was making her daughter's eyes swell shut: an entire class's worth of midday snacks, the Cheese Doodles with oily residue that ended up on the scissors, the cheese-and-cracker packages that contaminated the hands that played with hers. "I always wore a beeper, and I never went more than a town or two away from home," says DeRienzo, whose daughter kicked and screamed as she was put on the school bus every morning, fearful of the discomfort she would find at the other end. This year, her daughter's school has limited the foods that students are allowed to bring into the classroom. Snacks that are deemed unacceptable are put back in the lunchbox and replaced with a safe snack on hand. As the students enter the room first thing in the morning and again after lunch, all of them clean their hands with Handi-Wipes.