Fruit allergy or pollen-associated cross-allergy?

Fruit allergies are a great example for a food allergy that can be pollen related. If a fruit allergy is caused by an underlying allergy to pollen, it is called a secondary pollen-associated food allergy. If that is the case, the term “cross-reactivity” is often used to describe the process of the immune system reacting to an allergen that is similar in structure to another that a patient is already allergic to.

Primary food allergy vs. secondary food allergy

What is the difference between a primary food allergy and a secondary food allergy? The answer lies in the sensitisation pathway. When a person develops an allergy, there is a crucial phase in between encountering an allergen for the first time, and the first time this person shows allergic symptoms: the sensitisation phase.

Primary food allergy

A primary food allergy often occurs during infancy and is caused by direct contact between the allergen and the gastric mucosa – the membrane layer of the stomach containing the glands and gastric pits.

Allergens causing a primary food allergy are usually very stable to heat (= they are not destroyed when a food item is cooked or baked) and retain their structure while passing through the gastrointestinal tract during digestion. These two properties are essential for the sensitization phase as well as triggering true fruit allergy reactions. Examples for allergens possessing these properties are the apple allergen Mal d 2 and the kiwi allergen Act d 2.

Secondary food allergy

A secondary food allergy mainly affects children, adolescents and adults who have previously been diagnosed with a primary pollen allergy and consequently react to cross-reactive food allergens in fruits and vegetables. This is also called the pollen-fruit syndrome. Fruit and vegetable allergies result from a primary allergy to pollen. Cross-reactive food allergens are similar in structure to pollen allergens, and therefore usually cause mild and local reactions, also known as the oral allergy syndrome (OAS). Symptoms of OAS include redness, mild swelling or itching of the lips, tongue, inside of the mouth, soft palate, and ears, as well as itching and mild swelling of the throat. Occasionally, symptoms such as abdominal pain, nausea and vomiting can occur; as well as sneezing, runny nose, or itchy and watery eyes – symptoms known to be related to pollen allergy.

Class I vs. class II food allergens

To illustrate how molecular allergy diagnostics works to diagnose food allergies, apples can be used as an example. The apple allergen Mal d 1 is related to the Birch pollen Bet v 1. Since Mal d 1 is susceptible to heat and digestion, allergy patients who have elevated IgE levels to Mal d 1 can usually tolerate cooked or baked apples, and “only” have an oral allergy syndrome as far as symptoms go.

This is not the case with another apple allergen called Mal d 3. Patients with elevated IgE levels to Mal d 3 have a “real” apple allergy and show severe systemic reactions when eating apples, even when they have been cooked or baked.

In general, science divides food allergens into two categories: class I and class II allergens.

Class I allergens

Class I allergens are very stable allergens that withstand treatment with heat and digestion. Their chemical structure remains intact even when they are cooked or baked. Because of this property, they can sensitize a person through the gastrointestinal tract. They are therefore considered “true” food allergens, because they can trigger a primary sensitization and cause severe, non-pollen related allergic reactions by themselves.

Class II allergens

Class II allergens are proteins that are homologous (= they possess the same structural features) to other pollen proteins. They are proteins that cause the pollen-fruit syndrome and are often altered by heat during cooking or stomach acid during digestion. They are easily degraded, digestible, heat-labile, and therefore result in milder, local symptoms such as the oral allergy syndrome.

Cross-reactions with latex allergy

Although it is much less common, severe true fruit allergies are sometimes also associated with latex allergy. In this case, we speak of a latex-fruit syndrome. Some proteins in fruits and vegetables are similar in structure to pollen as well as latex allergens. Patients that suffer from a latex allergy may also react allergically when eating foods containing the corresponding allergens. As a result of latex-fruit syndrome, up to 88% of latex-allergic adults have specific IgE to plant foods.

Diagnosing fruit allergies

For a quick, easy, and most importantly, accurate diagnosis, modern allergy testing relies on molecular testing methods.

In vitro molecular allergy tests like the ALEX test offers a practice-relevant solution at the highest level. It includes both cross-reactive components as well as primary food allergens. This way, clinically irrelevant positive tests because of cross-reactive foods can be filtered out and patients can be diagnosed accurately.