If you’ve ever had an itchy mouth or burning lips after eating a piece of fresh fruit or some nuts, you may have experienced an allergic reaction. This is especially true if you have certain pollen allergies. How are pollen and food sensitivity connected? By proteins in both that elicit a similar reaction in patients with pollen allergies.¹

What is Oral Allergy Syndrome (OAS)?

Oral allergy syndrome, or pollen food allergy syndrome (PFAS), is caused by cross reactivity, which is when our bodies recognize proteins in one substance, such as pollen, as being similar to proteins found in another substance, like certain foods.²

For patients with an allergy to those pollen proteins, the body reacts when it encounters a comparably shaped protein in food. These proteins are known as class 2 allergens because they break down when heated or during digestion, so they may cause a reaction when they first touch someone’s mouth.³

Cross-reactivity caused by OAS is more common than you may think — it’s estimated that over 60% of food allergies are actually cross-reactions between food and inhaled allergens.¹

Oral Allergy Syndrome Symptoms

OAS symptoms are generally mild, but in certain cases OAS can result in life-threatening anaphylaxis, making it important for patients to understand their triggers.⁴ Allergy patients may not even think to tell their healthcare provider about these reactions to food because they seem unrelated to seasonal allergies.⁴

OAS symptoms can happen at any time of year, but if you have seasonal allergic rhinitis (AR), they may be more likely to happen during the pollen season of your particular trigger.² Studies have shown that patients with seasonal AR and/or asthma are more predisposed to have OAS — another reason why allergy patients should ensure they understand their triggers.

It’s also quite likely that OAS is under-diagnosed in children because kids may not accurately describe their symptoms, so parents assume they just don’t like certain foods.⁵

A specific IgE blood test is an important part of determining allergic triggers, so if you’ve experienced OAS symptoms, talk to your healthcare provider about testing and an allergy-focused medical history profile.

Certain plant families have similar-shaped proteins that cause cross-reactivity, making it vital to understand if you’re sensitized to certain pollens.

Birch pollen and cross-reactivity

Pollens from the Betulacaea family of trees include birch, alder, and hazel trees and are a common cause of cross-reactivity with food. In fact, studies have found that up to 90% of patients with a birch allergy also have OAS.⁶

People with these allergies often experience OAS when eating stone fruits, or more specifically, fruits from the Rosacaea family, which includes apples, pears, peaches, plums, cherries, and apricots. Kiwis, bananas, and strawberries also cause OAS symptoms, as do vegetables including potatoes, celery and carrots.²

Nuts are another important trigger of cross-reactions, particularly hazelnuts, almonds and walnuts. Legumes such as beans, peanuts and lentils, and even wheat, can cause OAS in patients with birch allergies.²

Alder trees are also a member of the birch family, and patients with alder pollen allergies can have similar OAS triggers — namely, stone fruits like apples, cherries, and peaches; other fruits and vegetables like strawberries and celery; and nuts including hazelnuts, almonds, and walnuts.²

Why are there so many cross-reactions between pollens in the birch family and other foods? Because those fruits and vegetables, or more specifically, the plants they come from, have proteins that are very similar to the allergy-inducing Bet v 1 protein in birch pollen.⁶ In addition to the Rosacaea family, the Apiacaea family, which includes celery and carrots, has proteins that are similar to Bet v 1.³

Weeds, grasses, and cross-reactivity

Sensitivity to weed pollen can cause OAS with kiwis and bananas, and weed pollen is also noted for cross-reactivity with the Cucurbitaceae family. This plant family includes many types of melons and also zucchini and cucumber. One study found that 50% of ragweed patients were sensitized to a member of this plant family.³

Patients with allergies to ragweed and other weed pollens may experience OAS from honeydew and watermelons. They may also have symptoms after eating tomatoes, cucumbers, or squash. For people with allergies to grass pollen, melons and tomatoes are also OAS triggers.²

Patients with ragweed pollen allergies may have cross-reactivity with sunflower seeds, but otherwise nut allergies are not associated with weed pollen allergies.³ Grass allergies, however, show an association with peanut cross-reactivity, although peanuts are technically a legume, not a nut.¹

Testing and diagnosis for Oral Allergy Syndrome

The proteins that cause OAS symptoms can usually be neutralized by cooking, except in the case of celery and strawberries.¹ This means that consuming raw food, as fruits typically are eaten, is when OAS symptoms are most likely to occur or are worse.

By getting a specific IgE blood test, you and your healthcare provider can build a fuller picture of your allergic triggers and develop a plan to manage them, which might include avoiding the food. For example, immunotherapy may be beneficial if a single allergen is implicated.¹

OAS can be a minor irritation or a more serious health risk, but with allergy testing and medical history information, you can equip yourself to avoid your triggers and still be able to enjoy fresh foods — and not the ones that will cause an itchy mouth.