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Not all that itches is allergy

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Allergens, especially from the plant kingdom, are often closely related at protein level, even if this is not always evident to the naked eye. Birch and peanut are for example fairly closely related, which means that people who are allergic to birch pollen can suffer mild symptoms when they eat peanuts. They may also have tested positive for peanut allergy in older tests.

“If you are allergic to birch, then you may have IgE antibodies against peanut, even if you are not allergic to peanuts,” says Dr Caroline Nilsson, paediatric allergist at Sachs’ Children and Youth Hospital and Docent at Karolinska Instiutet’s Department of Clinical Science and Education, Södersjukhuset (KI SÖS).

In Sweden, birch pollen allergy is fairly common, which means that there are a good number of adults today who were told by a doctor decades ago that they were allergic to peanuts, but who in reality may not be.

Erik Melén, photo: Anna Persson/IMM.“If you have been diagnosed with a peanut allergy as a child, and you haven’t been tested in years, it would probably be good to look into whether you still have a serious allergy to peanuts, if you have outgrown it, or if in fact it was only an allergy to birch pollen,” says Dr Erik Melén, paediatric allergist at Sachs’ Children and Youth Hospital, Docent at the Institute of Environmental Medicine (IMM) at Karolinska Institutet.

Several similar cross-reactivities exist; there are for example many people who are allergic to birch pollen who feel that their mouths itch if they eat an apple.

“This is usually called an oral allergy syndrome, the same thing that happens if you eat peanuts. If you have antibodies against this birch-like protein, you can feel itchy in both mouth and throat. But it is of course very difficult for a layman to know if this is the onset of a serious reaction or just a cross-reaction,” says Caroline Nilsson.

Broken down by heat

But the birch-like protein in apples is broken down by heat, which makes it possible to conduct a fairly simple, and tasty, diagnostic test at home.

Caroline Nilsson, foto: Fotogruppen Södersjukhuset“If you are able to eat apple pie, but not fresh apples, then you are not allergic to apples, but to birch pollen,” says Caroline Nilsson.

Birch pollen allergy can also give cross-reactions for hazelnuts, raw carrots and raw potatoes. Allergy to mugwort pollen gives cross-reactivity to vegetables and herbs in the same family, such as tarragon, sunflower seed, lettuce, artichoke and black salsify.

Latex, which comes from the rubber tree, can give cross-reactions to banana, avocado, edible chestnut and kiwi. A previously unknown correlation was discovered when a boy with a severe allergy to chicken was served crocodile meat as an alternative and had an anaphylactic reaction.

“As it turned out, the protein in chicken and the protein in crocodile are very similar. This is because they are actually distant relatives, from the time of the dinosaurs,” says Caroline Nilsson.

Text: Fredrik Hedlund , first published in Swedish in Medicinsk Vetenskap No 4/2018.