Pollen food syndrome (PFS), also known as oral allergy syndrome, is the most prevalent new onset food allergy in adults but can also affect children (link to PFS information sheet and PFS guideline when available). It occurs in people sensitised or allergic to tree pollen, or occasionally grass/weed pollens, who react to fruits, vegetables, legumes or nuts due to the similarity of the allergens. The UK prevalence in the general adult population is 2%, but around 2/3 of those sensitised or allergic to birch pollen may have the condition. In children, studies suggest 50% of those with seasonal allergic rhinitis will also have PFS. The key allergens provoking PFS are those which are homologous to the birch pollen allergen Bet v 1, and these include Ara h 8 (peanut) and Cor a 1 (hazelnut). Raw fruits and to a lesser extent, vegetables are also common causes of PFS, often provoking mild symptoms, although concentrated amounts of allergen such as freshly made fruit juices, smoothies, or soy milk, which contains the birch cross-reacting allergen Gly m 4, can cause severe reactions. The history of reactions can be diagnostic, as PFS is characterised by immediate-onset mild/moderate oropharyngeal itch and swelling, most often to raw fruits and nuts, especially apples, stone fruits, kiwifruit, hazelnuts, almonds walnuts and peanuts. Cooked fruits and vegetables are usually tolerated as are nuts in foods. Tests may be needed if nuts are involved, with CRD being best able to discriminate between a diagnosis of PFS or a primary allergy to nuts/soy (link to BSACI nut allergy guideline).
Although PFS is the major plant food allergy in the UK, there is another type of cross-reactive plant food allergy becoming increasingly prevalent. Known as lipid transfer protein (LTP) allergy, this condition was originally described only in South Mediterranean countries but more recently has been recognised in some northern European countries. It is more prevalent in adults who report reactions not only to raw plant foods but also cooked foods including composite dishes such as pizza. Another feature of LTP allergy, is that people might only react to the food when a co-factor such as exercise, alcohol or pain relief is present. The symptoms tend to occur within 30 minutes of eating foods containing fruits, vegetables, nuts or cereals, and can be on a wide spectrum from urticaria and oropharyngeal symptoms, to anaphylaxis. The clinical history can often be used to help discriminate between LTP allergy and PFS, however tests are also an important part of the diagnostic work up because the foods involved may be similar for both PFS and LTP. The marker allergen for the diagnosis of LTP allergy is Pru p 3, the peach LTP allergen. Sensitisation to this allergen can indicate LTP allergy even if peaches are tolerated. Other LTP allergens useful to test for including those from Walnut (Jug r 3), peanut (Ara h 9), plane tree (Pla a 3) and mugwort (Art v 3).