Hundreds of different allergens have been identified, and can be classified by similarities in structure/genetics, usually based on the plants being related to each other in evolutionary terms.
This is useful because cross reactivity is more common, the more closely different proteins are related. Not all are allergenic however, and not all cross react.
Looking at specific allergenic proteins (“components”) can also improve diagnostic specificity. For example, Peanut Ara h2 has higher sensitivity than whole peanut IgE at the same level of specificity –
- Ara h2 at 0.35 kUA /L cutoff has 83.3% [95% CI 75.6, 88.9] sensitivity, and 83.6% [95% CI 77.4, 88.4] specificity. [PAI 2020]
- At the same cut off, Ara h1 and Ara h3 had comparable specificity but significantly lower sensitivity.
You can also predict how heat stable these proteins are by how cross linked their structure is. Linear proteins are more easily disrupted by heat, so the allergy is likely to only be an issue with raw food, and the risk of anaphylaxis much lower.
Most allergens belong to one of a small number of groups:
- PR-10 eg Bet v 1, Ara h 8, Cor a 1, Pru p 1, Mal d 1. Heat labile, homologous. Most pollen food syndrome cases (birch pollen).
- Profilin eg Bet v 2, Cor a 2. About 20% of pollen food syndrome cases. More common in Southern Europe – birch and oak too, but also olive tree, London plane, grasses (eg Phl p12), ragweed. Citrus, raw tomato, banana, melon/watermelon, pineapple, courgette (but different from latex-fruit syndrome). Unstable with heat, as PR-10, so considered low risk, but reports of severe food reactions in Spain, apparently related to high grass pollen intensity (not citrus so much but the others), and some reports of severe reactions with co-factors.
- Prolamin – includes nonspecific lipid transfer proteins (nsLTP) which are heat stable but very cross reactive, and are found in fruit, vegetables, nuts, legumes, seeds and cereals. Best known is Pru p 3, which is a good surrogate marker for any nsLTP sensitisation, even if peach (prunus persica) isn’t a known issue. Ara h 9, Cor a 8, Jug r3, Mal d 3. Mugwort related pollen food syndrome is usually due to an LTP (Art v 4 with Api g 4 of celery and Dau c 4 of carrot, else Foe v 5 (fennel), Sin a 3/4 of mustard). Severe reactions possible.
- Cupin – includes legumins. Heat stable. Ara h 1 and 3, lentil, cor a 9/11 (hazelnut), some soya.
- Thaumatin – named after W African shrub! Various fruit including apple, kiwi, plus cedar pollen.
- 2S albumin eg Ara h 6.
Glasgow lab offers only hz, peanut, peach/cherry, egg (Gal d 1), alpha galactose and bee/wasp. Dundee offers those plus milk and cashew.