When the body confuses apple and birch, the result is cross reactivity

Autumn is apple season. Fresh, juicy, crisp and straight from the tree, apples are a delightful treat. Not so for people with apple allergies. Usually anywhere between five and 20 minutes after the first bite, they start to have symptoms: the lips, mouth and throat start to itch and burn, the tongue swells, the nose becomes runny and itchy. They can also develop breathing problems severe enough to make them feel as if they are suffocating. Nausea and diarrhoea may also occur. After an hour or two, everything is fine again.

Why apples?

Why does the body react to apples, which are nothing more than a healthy fruit? It’s actually all just a big mix-up that affects people who are already allergic to tree pollen. The immune system incorrectly identifies tiny components in the apple as an allergy-triggering substance in birch pollen (this can also happen with alder and hazelnut pollen). It responds with a type 1 hypersensitivity reaction.

When it comes to allergy-triggering substances, or allergens, many people think of pollen, cat hair or latex, which are all visible allergy triggers. The definition of allergen in a narrower scientific sense is something else – invisible and much smaller, such as a protein in pollen or food. This is also the case with the cross-reaction between birch pollen and apple.

Bet v 1 and Mal d 1

Birch pollen contains the protein Bet v 1, which appears to play a role in the defence against pollutants and in promoting plant growth (Marković-Housley 2003). The composition of Bet v 1 is always the same, but it can be assembled in different ways (isoforms). They differ from tree to tree, which is why individual birch trees trigger a varying severity of allergies (Schenk 2009).

Apples also contain plant stress proteins (pathogenesis-related (PR) protein families) that protect them against harmful substances. One of these proteins is called Mal d 1 (‘Mal’ from ‘Pyrus Malus’, the Latin word for cultivated apple). It is the main trigger of apple allergies in Central and Northern Europe and in North America.

Mal d 1 is very similar to Bet v 1. In particular, part of Mal d 1 looks the same as the site onto which IgE antibodies bind – the antibodies typical of a type 1 hypersensitivity reaction. The immune system only recognises the binding site, regardless of whether it is on the pollen or in the apple. In fact, 73 percent of people with birch pollen allergy experience ‘cross reactivity’ with apples (Geroldinger-Simic 2011).

Major differences between apple varieties

People with an apple allergy can still enjoy the taste of apples if they cook or bake them first, which destroys their allergenic properties. This is because Mal d 1 is sensitive to heat.

A lot of Mal d 1 is found the skin of the apple, and therefore peeling also helps.

In addition, fresh apples are generally better tolerated than stored apples.

However, new studies have shown that not all apples containing less Mal d 1 automatically trigger fewer allergies (Romer 2020). The amount of the secondary plant substance flavan-3-ol is also a contributing factor, as is the chemical structure of various forms of Mal d 1.

So what are the options?

People with allergies need to choose the right apple. But which is the right one and where can it be found? In the end, only two dozen out of the approximately 2,400 commercially available apple varieties are better tolerated.

Scientists from Charité Berlin, led by ECARF expert Karl-Christian Bergmann, conducted a pilot study in 2016 to investigate which apples allergy sufferers can tolerate. They also focused on old apple varieties.

They discovered that people with apple allergies are better able to tolerate these old varieties. In addition, the old apple varieties are healthier because they contain more vitamin C and phytochemicals. The environmental association BUND Lemgo has compiled a list of apple varieties containing information on how well they are tolerated.

The sourer, the better

One reason certain apples are better tolerated is because they contain polyphenol, which not only protects the apple against UV rays, fungal infections and insects, but also renders the allergens in the apple harmless. Polyphenols are plant substances that keep the apple aromatic but make it less sweet. So for people with an apple allergy, the sourer, the better.

Apples containing a lot of polyphenol turn brown faster when they are cut, so they look less than perfect. This makes them more difficult to sell, which is why new varieties contain less polyphenol than their predecessors.

With a bit of searching, the old varieties can be found at markets, organic food shops or directly from the producer. They are worth the trouble in any case.