The oral allergy syndrome (OAS) is caused by 'cross-reactivity' between similar proteins found in certain fresh foods (fruit, vegetables and nuts) and pollen (usually birch tree and grass pollen in the UK). The OAS is more common among teenagers and adults, but Dr Du Toit is noticing that children are now commonly affected even in their first decade of life. 

The OAS develops subsequent to the development of pollen sensitisation, hence tolerance of these foods is typical when younger. When children then start developing an aversion of these foods this confuses parents and often the aversion is seen as 'fussiness'. Confusing the clinical presentation further for parents and the patient alike is that the foods can be eaten when cooked e.g. tomato sauce is typically tolerated but not fresh tomato, or apple as apple juice or apple pie but not fresh apple. 

As the majority of the responsible allergens reside in the peel of the fruit, many individuals with OAS will tolerate the fruit when eaten peeled.  

The OAS is unlike a 'classic' food allergy, which develops independent of pollen allergy.  The responsible proteins in the food (fruits and/or vegetables and/or nuts) that cause the OAS symptoms are 'heat-labile' and are therefore easily destroyed by cooking and sometimes by freezing or processing of food. Very few patients outgrow the OAS although the severity of symptoms associated does vary and is typically most troubling during the relevant pollen season. 

Cross Reactivity between Foods

If children are allergic to one food, they may also show reactivity to additional foods. This arises either because they are multiple food allergic (to unrelated foods) or because they are reacting to foods which belong to the same biological family.  Seemingly unrelated foods may therefore contain the same allergen as another, e.g. peanut, tree nuts and sesame seeds.  It should never be assumed that being allergic to one such food will cause clinical reactions to other related foods.

The most well-documented cross-reactivity is the one which occurs between apple and birch pollen. Nevertheless, all apple allergics are not necessarily allergic to birch pollen.  The following are examples of claimed cross-reactivities based on studies with immunochemical methods and/or clinical diagnosis, some of these associations are stronger than others - the strong associations are bolded in black with highly relevant associations in red. The codes represent the Phadia codes for the allergy IgE test

Food Claimed Cross Reactivity
apple (f49) potato (f35), carrot (f31), birch pollen (t3)
carrot (f31) celery (f85), anise (Rf271), apple (f49), potato (f35), rye (f5), wheat (f4), birch pollen (t3), avocado (f96), pineapple (f210)
cereals (f4, f5, f6, f7) wheat (f4), rye (f5), barley (f6), oats (f7), maize/corn (f8), rice (f9), corresponding pollen, grass pollens
cod (f3) eel (Rf264), mackerel (Rf206), salmon (f41), trout (f204), tuna (f40), bass, dentex, sole, haddock
cow's milk (f2) mare's milk (Rf286), goat's milk (Rf300), cow's milk-based formulas (Rf228)
egg (f1, f75, f245) egg yolk (f75), egg white (f1), lysozyme (Rk208), ovalbumin (f232), ovomucoid (f233), inhaled bird antigens
garlic (f47) onion (f48), asparagus (Rf261)
pea (f12) soy bean (f14), white bean (f15), peanut (f13), lentil (f235), fennel (Rf219, Rf276), guar gum (Rf246), carob bean (Rf296), tragacanth (Rf298), chick pea (Rf309), liquorice
peach (f95) apricot (Rf237), plum (Rf255), guava (Rf292), banana (f92)
nuts walnut (f256) and pecan, cashew and pistachio
rice (f9) wheat (f4), rye (f5), barley (f6), oats (f7), maize/corn (f8), rye pollen
shrimp (f24) crab (f23), lobster (f80), squid (Rf258), crayfish (Rf320)


Most people with OAS experience mild to moderate intra-oral symptoms, such as a 'metallic' taste, itching, burning and tingling. Occasionally, swelling of the lips, mouth, face, tongue and throat may occur. Symptoms are usually short lived (a few minutes) and rarely progress to anything more serious. Occasionally, in highly-sensitive individuals, gut pain, vomiting, diarrhoea and/or a flare in eczema may be experienced even when the food is eaten cooked. Symptoms are usually more severe during the season in which the responsible pollen is at its height. The allergy specialist only rarely prescribes an injectable adrenaline device for use in the OAS.

Cross reactions have been described for the follwing foods. Importantly not all the foods listed below will cause reactions for any one individual:

  • Birch pollen (common in the UK), including hazelnut, apple, peach, pear, apricot, carrot, celery, cherry, chicory, coriander, fennel, fig, kiwifruit, nectarine, parsley, parsnip, pepper, plum, potato, prune, soy, wheat; Potential reaction: almond nut, and walnut.
  • Alder pollen, including almond, apple, celery, cherry, hazel nut, peach, pear, parsley.
  • Grass pollen (common in the UK), including fig, melon, tomato, orange.
  • Mugwort pollen (more common in USA), including carrot, celery, coriander, fennel, parsley, pepper, sunflower.
  • Ragweed pollen (common in the USA, and increasing in parts of Continental Europe), including banana, cantaloupe, cucumber, honey dew, watermelon, zucchini; Potential reaction: Dandelion or chamomile tea.
  • Possible cross-reactions (to any of the above pollens), including berry (strawberry, blueberry, raspberry, etc), citrus (orange, lemon, etc), grape, mango, fig, peanut, pineapple, pomegranate, watermelon.


  • Awareness and diagnosis of OAS.

  • Avoidance of the offending food, but only in extreme cases.

  • Eating well-cooked, canned, pasteurized or frozen foods, as these cause little or no reaction.
  • Peeling food has shown to reduce the effects of the allergy in the throat and mouth, especially in the case of apples. These measures may not always help prevent/relieve symptoms in the gastrointestinal tract or skin for those who are highly-sensitive.
  • Antihistamines may relieve the symptoms - especially during the pollen season.
  • SLIT (Sub-lingual Allergy Immunotherapy) may improve OAS.
  • Rarely, persons with severe reactions may consider carrying injectable medication, such as an EpiPen, to bring relief if necessary.

Further Information

UK Anaphylaxis Campaign Information sheet on OAS

Wikipedia Information on OAS