When Your Body Confuses Tomato with Grass: Meet Oral Allergy Syndrome

Your body reacts to carrots like its birch tree pollination season, and to bananas as if you’ve just run through a field of ragweed. You take one bite of that raw carrot stick and now your throat feels scratchy, your mouth is getting itchy, and even your ears are tingling. What is going on?

It might just be oral allergy syndrome.

What is Oral Allergy Syndrome (OAS)?

Also known as pollen-food allergy syndrome, OAS is a unique allergic reaction triggered by a misfiring immune system. Your immune system is befuddled enough, overreacting to grass, tree, and weed pollens as if they were dangerous invaders, but now with OAS, it’s so disorientated it thinks that certain proteins in fruits, vegetables, and spices are the same allergens. This is known as cross-reactivity – when your immune system confuses a protein with another one it’s previously identified as an allergen, sparking the same allergic reaction or exacerbating your existing allergy symptoms.

Unlike a regular food allergy, OAS causes problems only with raw foods. The process of cooking breaks down (denatures) the proteins, distorting their shape so that they no longer resemble a menacing tree, grass, or weed pollen. However, certain foods can be associated with both OAS and a true food allergy, such as kiwi fruit and nuts. The allergens in these foods can cause an allergic reaction even after cooking or processing, sometimes even resulting in anaphylaxis.

How Common is Oral Allergy Syndrome?

The prevalence of oral allergy syndrome varies widely across studies and may be influenced by the geographic location of the population studied. Researchers estimate OAS affects between 20-50% of the population. This number may be underestimated because mild symptoms go unnoticed and unreported, or because some doctors miss the connection entirely due to a lack of awareness of the condition.

Toddlers under the age of 3 years old are not likely to experience OAS as the development of allergic rhinitis (hay fever) typically only occurs in older children. Studies report the prevalence of oral allergy syndrome in children to range from 4.7% to 26.7%, with this variation likely due to the same factors that affect estimates in adult populations.

Oral Allergy Syndrome Symptoms

OAS is found in those with allergic rhinitis (hay fever), though not everyone with hay fever will get OAS. The most common characteristics of oral allergy syndrome are:

• Itchy mouth
• Irritated throat
• Swelling of the lips, mouth, tongue, and throat
• Symptoms that occur almost immediately after putting the food in your mouth

Less frequently, OAS can cause hives around the face, nasal congestion, a runny nose, sneezing, and itching around the nose or ears, and sometimes symptoms may only develop an hour after eating the food. In rare cases, OAS may result in more severe symptoms, including nausea and vomiting, stomach pain, airway swelling, and anaphylaxis.

Most Common Oral Allergy Syndrome Triggers

Up to 75% of people with an allergy to birch tree pollen will also have an OAS reaction to uncooked apples, almonds, carrots, celery, cherries, hazelnuts, kiwi fruit, peaches, pears, or plums. You may have an allergy to only one food on this list or, if you’re unlucky, to all the foods on this list. If you’re very special, you may have a reaction only to a specific variety, like Golden Delicious apples.

Those with a grass pollen allergy – and in Philadelphia, the most common are Timothy, rye, Bermuda, and Kentucky bluegrass – may find themselves itching and swelling after celery, melons, oranges, peaches, and tomatoes. And if you know you get hay fever from ragweed, you might want to be cautious with bananas, cucumbers, melons, and zucchini.

Management of Oral Allergy Syndrome

As with all allergic diseases, avoidance is key. Especially when it’s the season for the pollen in question – typically, birch pollens in spring, Timothy and orchard grasses in summer, ragweed in late summer to fall, and mugwort in fall – you can reduce your symptoms by:

• Cooking or heating the offending food, including microwaving or baking
• Choosing canned versions over fresh versions of the food
• Avoiding the food altogether
• Peeling the food (if it has a peel), as the triggering protein is often in the peel of the fruit or vegetable

There is also some evidence that allergy shots can help with oral allergy syndrome.

If oral allergy syndrome alongside allergic rhinitis has you feeling doubly miserable during hay fever season, see an allergist. This is particularly important if you’re finding you react even to cooked forms of the food, have systemic symptoms, or react to high-risk foods such as nuts.