Oral allergy syndrome

Allergy, General paediatrics, Immunology

Used interchangeably with Pollen food syndrome, although PFS is probably a better name because it is more specific and more closely represents what is going on.

Pollen Food Syndrome (PFS) refers to fruit and/or nut allergy, associated with birch pollen allergy (ie hayfever).  The food allergy part of the deal however is usually mild, eg itching/tingling/scratching/numbness, of lips/tongue/throat (sometimes ears) only, which distinguishes it from primary peanut or tree nut allergy. Peri-oral rash, nausea, abdo pain are sometimes reported. Mild angioedema of tongue and lips can occur, as can cough – you would be more nervous about assuming PFS in this situation, of course.

Peeling root vegetables is a trigger in those who have vegetable types of PFS! Another reason why PFS is a better name.

Extremely common – probably 40% of children with birch pollen related hay fever (and more than 70% of adults)! Getting more common too – climate change related hay fever season, and more allergenic pollen (thought to be relate to ozone levels, hence cities can be worse)…

Due to cross reactions between birch pollen and similar proteins (PR-10 group – also oak, beech, alder) found in fruit, particularly those fruit with pips inside and a peel eg apple, pear, peach – this group of fruit is known as Rosaceae and includes nectarine, cherry, apricot, plum.

Hazelnut and almond most common nuts involved in PFS in UK.  Sunflower seed can be involved.

Profilin group more common in Southern Europe. Birch and oak too, but also olive tree, London plane, grasses, ragweed.

There are other similar allergy syndromes including celery-spice-mugwort syndrome and latex allergy. Watermelon profilin cross reacts with melon, kiwi, peach.

Kiwi and banana still quite common primary allergies, rather than PFS. Can be both, of course!

You may find that you can eat the fruit if peeled, tinned, cooked or processed (eg jam). The ripeness, even the specific species, can matter too. Cold storage may increase the risk of reactions. Freezing doesn’t seem to make any difference.

Some affected individuals even put up with the itching because they prefer not to miss out on their favourite fruit!

Often emerges in later childhood or early adulthood, and only after hay fever develops. Only described in 1942! Seen in up to 25% of kids, but they don’t always recognise it themselves! Big variations geographically, obviously related to distribution of birch.

The main impetus for making the diagnosis is to select patients with a lower risk of anaphylaxis, even if nut allergic.  However, not always possible, and certainly not 100% reliable.  Thought that about 3% of PFS patients have systemic rather than just oral symptoms, and 1-2% will have anaphylaxis.  Potential confusion too because people who have primary IgE mediated peanut or nut allergy may of course also report isolated oral symptoms if low dose exposure.  In the original description of oral allergy syndrome, 50% progressed to systemic reactions!

So, important to differentiate –

  • primary IgE peanut or tree nut allergy who have only had oral symptoms so far (but are are risk of severe reactions, due to sensitisation to storage proteins, typically found in seed/kernel)
  • benign PFS, where allergic to Bet v 1, its homologues (eg Ara h 8, Cor a 1 – see peanut allergy) or other PR-10 or Profilin.  These proteins are usually in the pulp of the fruit. Soya (Gly m 4) is an exception where soya milk can cause severe reactions.
  • PFS with potential for systemic reactions where allergic to LTP (typically found in the peel)

BSACI peanut and tree nut allergy guidance is to go with typical history and only test if:

  • Atypical/severe reactions
  • reactions to processed rather than raw plants (including roast nuts)
  • Soya products other than soya milk
  • Tree nuts other than hazelnut, almond, walnut
  • Legumes other than peanut

BSACI guideline says beware younger children (under 8) with kiwi, melon or banana allergy, even if otherwise typical history – PFS unlikely.

Testing could be Prick to prick tests (important to get skin and pulp!), or else consider:

  • Pru p 3 for LTP allergy– if reacts to cooked food or other atypical features (even if not peach specifically!)
  • Gly m 4 if soya milk negative (PR-10 but can be severe reactions).  Gly m 5/6/8 if atypical
  • For hazelnut and almond, best to do prick to prick, for raw AND roast; if positive, do Cor a 1 (PR-10, so PFS) with 9 and 14 (which would suggest primary)
  • Ara h 2 if any standard tests for peanut positive (should be negative in PFS) – if negative, Ara h 1/3/6
  • Jug r 1 if walnut tests positive, Ber e 1 for brazil nut.

Oral challenges only possibly needed if avoiding multiple foods.

Pollen subcut immunotherapy does not seem to help PFS, unfortunately.  Insufficient evidence about pollen sublingual immunotherapy.  Some work around oral immunotherapy with fresh food or with isolated proteins.

The risk of anaphylaxis in PFS is quoted as 10%, but this is perhaps misleading as it usually relates to very high doses such as smoothies, tofu, soya milk. Other possible risk factors include:

  • severe seasonal asthma
  • Acid suppression therapy!
  • Jackfruit!

Adrenaline autoinjectors are rarely justified.

But there are reports of anaphylaxis to peanut despite being exclusively Ara h 8 sensitised and passing an oral challenge – but had big dose on empty stomach! [https://doi.org/10.1111/cea.12425]

Golden Delicious, Granny Smith and Cox’s the worst of the fruit, with the most Mal d 1 (Bet v 1 homologue).

BSACI does not mention whether prick to prick with frozen is effective, only that you can still react to frozen fruit/veg.

More severe reactions described with jackfruit! Beware smoothies, protein shakes, edamame beans!

Bean sprouts, mange tout and sugar snap mentioned specifically as usually only lightly cooked, or just raw.

[Isabel Skypala, BSACI PFS guidance, Clin Exp Allergy 2022]