By KAREN ANN CULLOTTA


CHICAGO - For 5-year-old Sean Batson, even a grandmother’s kiss is to be feared.

“My mother was wearing lipstick, and when she kissed Sean’s cheek, it broke out in hives,”said his mother, Jennifer Batson.

At his first birthday party, Sean had a severe allergic reaction - hives, swollen eyes, vomiting and wheezing - to his first nibble of cake. And when a toddler with an ice cream cone touched Sean’s arm with sticky hands during a play date, the arm erupted in hives.

The daily struggle of living with Sean’s allergies to nearly unavoidable foods and food products - soy, eggs and milk, traces of which can turn up even in nonfoods like lipstick - prompted Mrs.Batson and her husband, Tim, to participate in a project that scientists are calling the most comprehensive food allergy study to date.

The international study, led by Dr.Xiobin Wang and Dr.Jacqueline A.Pongracic of Children’s Memorial Hospital here, is searching for causes of food allergy by looking at hundreds of families in Boston, Chicago and Anhui Province in China.

Using questionnaires and interviews, the investigators are gathering data on a broad range of environmental, genetic and health factors, among them diet, hygiene, number of pets and the children’s prenatal and postnatal medical histories.

Dr.Wang says the study’s design allows researchers to look at startling variations in the prevalence and types of food allergies across diverse populations and regions.

In China, for example, skin-prick testing found that large percentages of one rural population were sensitive to shellfish (16.7 percent) and peanuts (12.3 percent). Yet actual food allergies in that population, as diagnosed by physicians, were all but unheard of: less than 1 percent.

In the United States, by contrast, 12 million people (4 percent of the population) suffer from food allergies, according to the Food Allergy and Anaphylaxis Network, a nonprofit information and advocacy group.

“What can explain the U.S. and China difference?”asked Dr.Wang, director of the Smith Child Health Research Program at Children’s Memorial.“Is it urban versus rural exposure? Diet and lifestyle? Or genetic susceptibility? These are all questions we are trying to find some clear answers for.”

Dr.Wang said her own awareness of food allergies was heightened after her twin sons started kindergarten in Boston and began bringing home an abundance of notes warning of food allergies among their classmates.

“Going back 10 to 15 years ago, during my pediatric residency training, there was very little education about food allergies,”said Dr.Wang.

Indeed, with recent data showing a marked increase in the number of food allergies, which cannot be explained by a lack of detection in years past, the National Institutes of Health have begun an initiative to address food allergies as an emerging health challenge.

Although it is possible to be allergic to any food, eight foods account for 90 percent of all reactions - milk, eggs, peanuts, fish, shellfish, soy, wheat, and tree nuts like cashews and almonds. The most severe reaction, food-induced anaphylaxis, can be fatal.

Some experts suggest that children in a culture smitten with antibacterial detergents and hand sanitizers are exposed to fewer germs, depriving the immune system of its germ-fighting job and leading it to misidentify certain foods as foreign.

But that is still just a hypothesis, and many researchers say the causes of food allergies are highly complex, and the“hygiene hypothesis”cannot be the sole explanation.

Dr.Pongracic, who has been treating children with food allergies for 17 years, says even trace amounts of allergens can cause life-threatening reactions.

“Ultimately, we hope that our research will lead to the discovery of ways to predict which child is likely to outgrow food allergy,”she wrote in an e-mail message, adding that doctors hoped to develop therapies“that can lessen the severity of an allergic reaction, and even protect against the reaction in the first place.”