Cross-reactivity may sound like a description of your grandma’s irritation every time you walk through her pansies, but that’s not quite what it is.
As I mentioned in my previous post, 40% of those with pollen allergies have an associated plant food allergy due to cross-reactivity. Further, cross-reactivity is the predominant reason food allergies pop up in adulthood. While oral allergy syndrome is by far the most common way cross-reactivity presents, dangerous anaphylaxis can occur as well. Still, cross-reactivity perplexes both allergy sufferers and practitioners, which is why it will be the sole subject of this part in my allergies series.
What is cross-reactivity?
Cross-reactivity occurs when someone becomes sensitized to a particular allergen but then also reacts to separate species because of protein similarities.
Why do some proteins cause cross-reactivity reactions and others don’t?
As previously discussed, the bulk of food allergies are caused by proteins, and proteins are comprised of amino acids. One of the ways IgE antibodies recognize allergens is through amino acid sequences. If an unrelated protein has enough sequences in common with a substance we are allergic to, our IgE antibodies may flag it as an intruder as well. Yes, it may set our IgE Spidey sense and tongue tingling.
Why would two proteins be similar?
There are a couple reasons for likeness. Proteins from related species may share some components due to common genetics, but proteins from distinct species may also have commonalities because of similarities in function.
For instance, profilins are a group of proteins that are found in every plant part from fruit to nut. They perform essential functions like cell movement and, as they are present in almost every pollen, are a common pollen allergen. Roughly 10-30% of those with pollen allergies are sensitive to this family of proteins. Unfortunately, the uniformity between profilins from different plant sources is as high as 70-85%. What does this mean? Many people with pollen allergies will react when they encounter these proteins in food. Hence, profilins are involved in a high percentage of cross-reactivity cases.
Does the inhalant or food allergy come first?
Respiratory allergies are much more common than food allergies, impacting 10-30% of the world’s population. Therefore, in cases of cross-reactivity, the inhalant allergen is more often the primary sensitizer.
What are some of the common plants and foods that cause cross-reactivity reactions?
Which pollens and foods can cause cross-reactivity can be a bit confounding as the connections are not often obvious. Birch pollen and apple are the most frequently cross-reacting pollen and fruit pair. Birch pollen also causes cross-reactivity with carrots, pears, potatoes, and nuts.
Profilins, the protein family mentioned earlier, have been implicated in cross-reactivity with kiwis, apples, tomatoes, soybeans, celery, poppy seeds, and legumes. Fortunately, profilin allergy symptoms are typically mild involving a runny or stuffy nose in the case of pollens and oral allergy syndrome in the case of foods. On a side note, profilins are one of the allergens in birch pollen but are not the most common cause of birch allergies or cross-reactivity.
Many of those with a latex allergy will react to chestnuts, bananas, and avocados. This is because the protein involved in latex allergy, hevein, has 70% identical sequencing to proteins found in these fruits.
But cross-reactivity doesn’t stop with flora. People with a dust mite allergy can react to shrimp or other shellfish like mollusks.
Hopefully, how cross-reactivity connects the dissimilar is less confusing now. Next time, we will examine some of the properties of proteins and people that make allergies more likely to occur.
References:
1. Mills and Shewry. (2004). Plant Food Allergens. Blackwell Publishing.
2. Popescu. (2015). Cross-Reactivity Between Aeroallergens and Food Allergens. World Journal of Methodology, 5(2), 31–50.