The letters, marked URGENT, came home with children on the first day of school last week, with a lengthy list of dangerous foods. PLEASE, the principal wrote, stop packing lunches with any of these items. They could be fatal for a new first-grader. At the top: peanut butter.

Killer peanut butter? some parents at Stedwick Elementary School in Gaithersburg snorted. Yeah, right. Then the phones began ringing. Some parents were sympathetic. Many were confused. But most were angry, arguing. What harm could a PBJ sandwich do?

"The reaction surprised me," said Principal Joseph C. Rowe, who will meet with PTA parents this week to better explain what pupils should and shouldn't bring to school. "I did not expect this type of outrage."

Passions run high when it comes to the gooey spread the American Peanut Council estimates sits in the cupboards of 75 percent of the nation's homes and is packed in millions of lunch boxes every day. But as the number of children with a severe, life-threatening peanut allergy appears to be increasing, Stedwick is only the latest school to consider the sticky question: to ban or not to ban.

"Virtually everything in my house besides fruit is on that list," said Stedwick PTA President Kim Hardin, who has been fielding puzzled and peeved calls. In addition to peanut butter, the list includes chocolate, hydrolyzed plant protein--found in many cookies and crackers--and any baked goods. "What does baked goods mean? Does that mean bread? I've never heard of something this severe."

Meet Nicholas Allen, the reason for the letters to the parents of Stedwick's 530 students. He'll tell you.

Nicholas, with a blond crew cut and round glasses and wearing a beloved Pokemon T-shirt, is 6 years old. And peanuts could kill him.

"At school, the teacher said 'This is not a joke,' " Nicholas said as he administered pretend shots to his sister Marian's Beanie Baby. "It's not a joke. It's serious."

When he was still in diapers, his mother, Heather, gave him half of a peanut butter sandwich, often a staple in the diet of 1- and 2-year-olds. Within minutes, he had hives all around his mouth. She tried again a few weeks later. This time, hives broke out across his face and his eyes swelled shut.

"We took him to the pediatrician to get a blood test for peanut allergy. They said he was off the charts," Allen said. "Since he was 18 months old, I've carried an EpiPen."

EpiPens are Adrenalin shots of epinephrine, administered to the thigh. And the school nurse is training the staff how to use them and to recognize when Nicholas may be going into anaphylactic shock: swelling, vomiting, dizziness, hives, even falling into a coma.

It's happened before. At a party when he was 3, Nicholas grabbed a peanut butter cookie. Although his father, Jim, immediately brushed it out of his mouth, within minutes he was vomiting wildly. His ears swelled to twice their size. He stopped breathing.

They gave him a shot of epinephrine and rushed him to the hospital. Two more shots in the emergency room finally stabilized him. He spent the night in Pediatric Intensive Care with IV tubes pumping steroids into his little body and a nebulizer helping him breathe.

"I know it sounds dramatic, and I know that people don't understand the magnitude of this," Heather Allen said. "Even Nicholas's grandmother said it was the craziest thing she'd ever heard of. We said: Come to the hospital and see the tubes coming out of him. This is for real, and it's something we can't change."

Nicholas is one of an estimated 3 million Americans with peanut or tree nut allergies, about 1 percent of the U.S. population. Each year, about 125 people die from severe allergic reactions, and researchers say peanut allergy accounts for about half those deaths.

And the numbers are increasing, primarily among suburban populations of higher socioeconomic status. Scientists don't know why. The latest theory is that children are exposed in utero or through breast milk to more peanuts, peanut butter and peanut oil than ever before. Research shows the highly allergenic protein in peanuts passes through the placenta and through breast milk.

This is a tough theory for Allen. Violently ill through her pregnancy with Nicholas, peanut butter was the only thing she could keep down. She ate jar after jar. While pregnant with daughter Marian, now 4, Allen abstained. Marian has no allergies. "It's heartbreaking to think about," she said.

Another theory about the rise in peanut allergies is that the Western world is a very clean place. Houses are tightly insulated, children are vaccinated, and modern medicines kill harmful bacteria.

"Maybe because the body is not having to deal with infection, it looks for something to do," said Hugh Sampson, chief of the division of Allergy and Immunology at the Mount Sinai School of Medicine in New York. "So it attacks allergens," such as peanut protein.

China, the largest peanut-producing nation, has virtually no cases of peanut allergy. Nor do most of the Southeast Asian countries such as Thailand and Singapore, where peanut sauce is a staple with every meal. "Clearly, we're doing something here that's boosting incidence," Sampson said.

Sampson is part of a team working to develop a vaccine and other treatments for peanut allergy. Children never outgrow peanut allergy, as they can other allergies. The best they can do is avoid eating, breathing or in any way coming into contact with peanuts, peanut dust, peanut butter, peanut oil or any processed food that contains even a trace amount of peanut product. Allen spends hours reading ingredient lists on food packages and calling manufacturers.

Some airlines are being lobbied to ban packaged peanuts. But schools can be the most dangerous places. Of six fatal food allergy reactions in one year that Sampson studied, four occurred in school.

That's partly why Nicholas spent the first day of school in tears. He was afraid. Afraid of the monkey bars that might have a smear of peanut butter. Afraid of the pencil sharpener. Afraid of the doorknob on the classroom door.

"I cried because I didn't want to go outside," he said, munching on a peppermint patty, a favorite peanut-free treat. "I think maybe all the kids washed their hands. But I don't know for sure." He recites by rote how he is to turn down any food that anyone offers him. He is eager to read so he can inspect food labels himself.

So what is a school to do?

"The kids are all worried that if they eat something, they could inadvertently hurt [Nicholas] and they'd be responsible," said one Stedwick parent who asked not to be identified. "That is a big issue."

Some, mostly private schools in New York and elsewhere, have caused an uproar by banning peanuts and peanut butter outright. Other schools do nothing at all.

At Stedwick, principal Rowe has set up a special "no peanut" table where Nicholas eats lunch. Friends are encouraged to join him, but only after a teacher checks to make sure there are no peanuts tucked among the turkey, yogurt, bologna or pretzels. Maintenance workers are instructed to specially clean Nicholas's table, near the exit in case an attack comes on.

"We don't want him eating alone," Rowe said. "That's traumatic for any first-grader."

The school must accommodate Nicholas's special needs under the Americans with Disabilities Act, but Stedwick is not contemplating a ban. "We're asking parents if they can change a bag of peanuts for a bag of carrots. But if they can't, they can't," Rowe said. If a child will eat only PBJ, fine. School officials will keep that child away from Nicholas, he said.

"If we overreact, kids will pick up on that and become anxious," said Rowe, who is struggling to deal with such a case for the first time. "We're trying to keep it low-key."

While Nicholas said he would feel "more better" if peanut butter and peanuts disappeared from school, his mother knows that will never happen. Moreover, it probably shouldn't. "A ban can give you a false sense of security," she said.

She even debated at one point whether to home school her son. "But I want to teach him to live in a society that has things that are harmful to him, rather than shelter him," she said. "I want him to grow up to be a normal adult. He has to learn how to cope."