BioEd Online

Potentially lethal allergic reactions are much more common in the northern United States than in the south, researchers have found. What links geography and allergy is unknown, but the team behind the discovery suggests that sunlight might be a factor.

Extreme allergic reactions, commonly called anaphylaxis, result when the immune system overreacts to a usually harmless foreign substance, such as seafood, insect venom or peanuts.

Such reactions are becoming more common, but little is understood about their causes, partly because of the diversity of allergens and allergic reactions, which can include hives, vomiting, diarrhoea and respiratory distress.

Carlos Camargo at Harvard School of Public Health and his colleagues estimated the distribution of anaphylaxis in the United States by collecting information from all 50 states about prescriptions of the adrenaline auto-injectors used to treat attacks.

Some northern states such as Massachusetts and Connecticut had more than four times more prescriptions of adrenaline, at nearly 12 per 1,000 people, than southern states such as Hawaii and New Mexico, they report in the Journal of Allergy and Clinical Immunology¹.

"We were very surprised by the large difference, but previous research has shown that a related disease, asthma, follows a latitude gradient too, so the results were not completely unexpected," says Camargo.

Lack of sunlight

How geography affects anaphylaxis is unknown. The team looked at many different factors, including the number of allergy specialists per 1,000 people and the frequency of medication prescription in general, but none of these explain the north-south difference.

Camargo suspects that in the north a lack of vitamin D from sunlight may lead to the development of anaphylaxis. "We know that vitamin D affects the immune system in many different ways and that vitamin D is much harder for humans to obtain at higher latitudes, so this is something we need to look into more closely," adds Camargo.

"Connecting vitamin D and its effects on the immune system is an interesting hypothesis that would benefit from being rigorously tested," says Roger Katz, an allergy specialist at the University of California, Los Angeles.

Given how little we know, all ideas are welcome, he says: "There certainly are other possible explanations for these results, but thought provocation is what ultimately gets things done."

"We know that other explanations for the north-south gradient are going to arise, and we are open to exploring them too," agrees Camargo. "An important next step is to see if our finding can be replicated in other countries at similar latitudes."