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Cross-reactivity occurs when your body's immune system identifies the proteins in one substance (e.g. pollen) and the proteins in another (e.g. fruit & vegetables) as being similar. When you come into contact with either, whether it’s a protein in something that you are truly allergic to or not, your immune system can react in the same way, which can then cause your allergic symptoms.
All that matters, to your immune system, is that the proteins are structurally similar or biologically related and cross-reactivity can occur. The risk of cross-reactivity needs to be assessed by a healthcare professional, and important foods should never be removed from a diet without a confirmed diagnosis.
Cross-reactivity occurs when your body's immune system identifies the proteins in different substances as being structurally similar or biologically related
There’s a high degree of cross-reactivity between cashew and pistachio1 and between walnut, hazelnut and pecan,2 so some healthcare professionals may advise you to avoid all tree nuts even if you only have a confirmed allergy to one.
There’s also a high degree of cross-reactivity between different fish species,3 so healthcare professionals may advise their fish-allergic patients to avoid all fish.
And studies have found that the risk of an allergic reaction to goat's milk or sheep's milk in a person with a cow’s milk allergy is about 90%.4
A common cross-reaction is between birch pollen and stone fruit/kernel fruit such as apples, cherries and pears. If you have a pollen allergy and have experienced an itchy mouth or throat after eating fresh fruit or raw vegetables, you may have Oral Allergy Syndrome (OAS).
OAS, sometimes also called Pollen-Food Allergy Syndrome (PFAS), is usually a temporary and relatively mild condition due to cross reactivity. Some plant foods contain proteins that are similar to the proteins in plant pollens, so for people with OAS, their immune system mistakes the food proteins for the pollen proteins, and they’ll then have a reaction. It is important to remember that not everyone who is allergic to birch pollen reacts allergically to stone and kernel fruits.
In OAS, the immune system mistakes food proteins for pollen proteins, causing an allergic reaction
Cross-contact happens when a small amount of another allergen gets into a food that it shouldn’t be in. You may be familiar with the food labels that read “may contain” or “processed in the same facility as”―that's due to cross-contact.
Cross-reactivity is when two different allergens have a similar makeup at a molecular level, causing a person’s immune system to react to both allergens. For example, if someone is allergic to birch trees, they may also have symptoms when they eat apples because there are proteins in each that look identical and the immune system sees them the same.
When it comes to food allergies, it’s important to understand the difference between cross-contact and cross-reactivity to avoid confusion or an unnecessary reaction.
Not every patient with a pollen allergy will experience a cross reaction with food. Explore below to find the most common culprits of OAS-related reactions.