WASHINGTON March 31, (Reuters) -- Researchers said Tuesday they had taken the first step toward making a vaccine against peanut allergy, the most common cause of death from food allergies.

     They said they used DNA from peanuts to help de-sensitize mice to the allergens in peanuts, and said their approach might be used to develop a vaccine for humans.

     ``Although peanut allergy is an increasingly important public health problem, its only proven treatment consists of educating patients to completely avoid all possible sources of peanuts,'' Kam Leong and colleagues at Johns Hopkins University in Baltimore wrote in a report in the journal Nature Medicine.

     But they noted that up to half of all cases of severe reactions were due to accidental exposures.

     Airlines are even considering limiting the peanut snacks they commonly give out because those who are severely allergic can have a reaction from breathing in peanut dust released when passengers tear open the little foil packets.

     The American Academy of Allergy, Asthma and Immunology says between one half and one percent of all Americans are allergic to peanuts, and they say most of the 125 deaths every year from food allergies are due to peanut allergy.

     This could mean that peanut allergy is on the increase, because the Centers for Disease Control and Prevention (CDC) reported only 88 deaths among all Americans from food allergies, including peanuts, from 1979 to 1995.

     An allergic reaction to peanuts is marked by hives, wheezing, and diarrhea in its milder form, leading sometimes to a sudden drop in blood pressure and even death.

     Leong's team took the gene most blamed for causing this severe reaction, the pCMVArah2 gene, and mixed it with chitosan, a product from the shells of crabs and other crustaceans that is used for controlled drug delivery.

     They formed the mixture into tiny ``nanoparticles'' and fed them to mice specially bred to be allergic to peanuts.

     The mice showed an immune response to the vaccine, and when later injected with the protein produced by this peanut gene, these mice had a delayed allergic response.

     In contrast, allergic mice that did not get the immunization showed classic anaphylactic (allergic) shock immediately and some even died.

     Leong's team said their experiment had not been tried on humans, but that the tests on mice showed the idea might work.

     ``We chose to use a preventive vaccine model instead of a treatment model,'' they wrote.

     A treatment or therapeutic vaccine is used to treat existing disease, or in this case, existing allergy, as opposed to trying to prevent infection or allergic reaction in the first place.

     A vaccine might not work in someone who is already allergic to peanuts, they said.

     ``However, given that the only treatment of food allergy is complete avoidance of all allergen-containing food products and often involves aggressive emergency treatments, pre-immunization may be a viable therapeutic model,'' they wrote.

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