How a birch tree can make your mouth itch when you eat celery
Birch pollen contains a major allergen called Bet v 1.
Celery contains a protein called Api g 1.
The names look like obscure laboratory codes, but the important thing is their shape.
Api g 1 belongs to the same broad protein family as Bet v 1, and the molecules have enough structural similarity that IgE directed against birch pollen can sometimes recognise celery too.
This is cross-reactivity.
Imagine somebody who is very good at recognising a particular person's face.
Then they meet that person's sibling.
The sibling isn't identical, but enough features are shared that recognition occurs.
IgE can behave in a conceptually similar way with related proteins.
Research has demonstrated both structural and immunological relationships between Bet v 1 and Api g 1, helping explain the well-established association between birch-pollen allergy and celery reactions.
The immune system may therefore have become sensitised primarily through pollen breathed in during the pollen season.
Celery then becomes the molecular lookalike encountered at dinner.
Pollen-food syndrome
This type of reaction is commonly described as pollen-food syndrome.
Older sources may call it oral allergy syndrome, although pollen-food syndrome describes the underlying relationship more clearly.
In the UK, Anaphylaxis UK says pollen-food syndrome is the most common cause of allergic reactions to celery.
Typical symptoms can include:
itching or tingling of the lips or mouth;
mild swelling of the lips, tongue or inside the mouth;
an itchy or mildly swollen throat;
and sometimes symptoms affecting the nose, eyes, oesophagus or stomach.
For many people, symptoms remain localised and relatively mild.
And there is a fascinating reason cooked celery may sometimes be tolerated.
Why cooking can matter — but only sometimes
Some pollen-related food allergens are structurally fragile.
Heat changes their three-dimensional shape.
And shape is important because IgE doesn't simply read the chemical ingredients of a protein as though they were letters on a page. Recognition depends partly on molecular structures.
If heating unfolds or disrupts the relevant structure sufficiently, IgE may no longer recognise it as effectively.
Api g 1, the celery protein related to birch Bet v 1, is one of the allergens for which heat can make an important difference.
This helps explain why somebody with pollen-food syndrome may react to raw celery or celeriac yet tolerate it when thoroughly cooked.
But this is where an apparently useful rule becomes dangerous if stretched too far.
"I react to raw celery, therefore cooked celery is safe" is not a universal celery-allergy rule.
Celery contains more than one allergen.
Different patients recognise different proteins.
Some people react to cooked celery.
And a history of reacting to cooked celery is one of the features Anaphylaxis UK highlights when discussing risk of more serious reactions.
Cooking changes proteins.
It does not issue a safety certificate.
Mugwort changes the story again
Birch isn't celery's only interesting pollen relationship.
Mugwort is a widespread plant whose pollen causes seasonal allergy in many people.
Celery allergy has long been associated with mugwort sensitisation, particularly in parts of Europe.
Historically, clinicians have talked about patterns such as celery–mugwort syndrome and the broader celery–mugwort–spice syndrome, reflecting cross-reactivity involving celery, mugwort and some herbs or spices.
This is one reason celery allergy can become diagnostically complicated.
A patient may react to celery and also have sensitisation to other members of the botanical or molecular neighbourhood.
That does not mean everyone with celery allergy should remove a huge list of herbs and vegetables from their diet.
It means the protein pattern behind the allergy can matter.
And recent research has made that considerably more interesting.
Api g 7: the celery allergen you may not have heard of
Celery allergens are given systematic names beginning Api g, from Apium graveolens.
Researchers have identified several of them.
But one of the most interesting recent developments is Api g 7.
Api g 7 belongs to a family of proteins known as defensin-like proteins. It has a close molecular relationship with Art v 1, the major mugwort pollen allergen.
A 2024 study investigated 79 people with confirmed celery allergy. Thirty had relatively mild oral or eye/nose symptoms, while 49 had systemic reactions.
Api g 7 sensitisation was detected in about 52% of the patients.
More strikingly, sensitisation to Api g 7 was associated in that study with roughly six times greater odds of a severe anaphylactic reaction rather than only mild oral symptoms.
There was another important diagnostic finding: some people sensitised only to Api g 7 tested negative to conventional celery extract.
A 2025 review highlighted the same problem: Api g 7 may be clinically important, particularly in celery–mugwort allergy, yet current commercial diagnostic testing does not routinely capture it.
This is exactly the kind of development that shows why allergy information needs updating.
A page written ten years ago could still be broadly correct about celery allergy while missing a protein that may help explain both diagnostic blind spots and severe reactions.
It is also why EatSafe should never treat an allergy article as finished forever.
Not all celery allergy is the same allergy
This brings us to an important idea.
Two people can both say:
"I am allergic to celery."
Yet the molecular basis of their allergy may differ.
One person's reaction may be dominated by birch-related Api g 1 sensitisation and produce mainly oral symptoms with raw celery.
Another may recognise proteins associated with mugwort cross-reactivity.
Another may recognise more stable celery allergens and react to cooked food.
The label celery allergy describes the food involved.
It does not necessarily describe one identical immune condition shared by every patient.
This is increasingly true across allergy medicine.
We are moving from:
Which food?
towards:
Which molecules within the food?
That can help explain why patients with apparently the same allergy sometimes have very different experiences.
Celery and the spice cupboard
Celery belongs to the Apiaceae family.
Its botanical relatives include foods and herbs such as:
carrot;
parsley;
parsnip;
fennel;
coriander;
caraway;
anise;
and others.
This botanical relationship matters because some of these plants contain proteins sufficiently similar for the immune system to recognise more than one of them.
But — just as we discovered with mustard — botanical relatives are not automatically allergic relatives for every individual.
Someone with celery allergy should not look at the Apiaceae family tree and immediately remove carrots, parsley, parsnips, fennel, coriander and every related spice from their diet.
What matters is which proteins their immune system recognises and whether eating the related food actually produces symptoms.
And with celery, this becomes particularly interesting because one of its strongest allergic relationships isn't with another vegetable at all.
It is with pollen.
When celery allergy starts with a tree
In parts of Europe, celery allergy is strongly associated with birch-pollen allergy.
That sounds peculiar until we return to the molecular principle we established in the Mustard guide:
The immune system does not see tree and vegetable.
It sees molecular structures.
Birch pollen contains a major allergen called Bet v 1.
Celery contains a related protein called Api g 1.
The two proteins are sufficiently similar that IgE originally sensitised to birch pollen can sometimes recognise the celery protein as well.
This is cross-reactivity.
So somebody's allergic story may begin with springtime pollen floating from birch trees and eventually reveal itself when they eat celery.
This type of relationship sits within what is commonly called pollen-food syndrome, sometimes also known as oral allergy syndrome.
For some people, symptoms are predominantly itching, tingling or mild swelling of the lips, mouth and throat after eating particular raw plant foods.
The food hasn't somehow acquired birch pollen.
The immune system is confusing structurally related proteins.
That is one of the stranger facts about food allergy:
You can react to a vegetable partly because your immune system first learned to recognise a tree.
Mugwort enters the celery story too
Birch is not celery's only important pollen connection.
Mugwort pollen is also associated with celery allergy and with broader patterns of reactions involving spices and Apiaceae foods.
This has led to terms such as celery–mugwort–spice syndrome and celery–birch–mugwort–spice syndrome.
The names sound like somebody has simply listed the contents of an unusual allotment.
They describe genuine patterns of molecular cross-reactivity.
People sensitised to mugwort may show reactions involving celery and, in some cases, foods or spices such as carrot, parsley, coriander, fennel, caraway or anise.
Again, this is not an instruction to avoid the entire list.
It is an explanation for why a person who reacts to celery may sometimes report apparently unrelated problems with particular herbs, vegetables or spices.
If that happens, the pattern is useful diagnostic information for an allergy specialist.
Celery allergy is not one single molecular disease
This is where celery becomes considerably more interesting than a simple label saying:
Allergic to celery.
Several celery allergens have been characterised.
They include proteins belonging to different families, with different stability and different patterns of cross-reactivity.
Api g 1 is related to the birch-pollen allergen Bet v 1.
Api g 4 is a profilin — part of another widely distributed family of plant proteins involved in pollen-food cross-reactivity.
Api g 2 and related celery lipid-transfer proteins belong to the non-specific lipid transfer protein, or nsLTP, family.
Other celery allergens have also been identified, and research continues to refine our understanding of which molecular components are most important in different populations.
Why does any of this matter to somebody eating dinner?
Because different proteins can behave differently when food is heated or digested.
The word celery allergy describes the food involved.
It does not necessarily tell us which molecular route brought the individual there.
Raw celery and cooked celery can be different questions
This is particularly relevant to pollen-food syndrome.
Proteins related to Bet v 1 are often relatively vulnerable to heat and digestion.
Consequently, some people whose celery reactions are driven predominantly by pollen-related cross-reactivity may react more readily to raw celery than to extensively cooked celery.
That pattern is familiar across several pollen-related plant-food allergies.
But this is where a little knowledge becomes dangerous if turned into a universal rule.
Celery contains more than one allergenic protein.
Not all celery allergy is simply birch-related pollen-food syndrome.
And importantly, studies have shown that celery allergenicity can persist after heating, with particular celery allergens showing substantial heat stability.
So:
"Cooked celery is safe if you have celery allergy" is not a safe general statement.
Someone who tolerates cooked celery but reacts to raw celery may have a clinically meaningful pattern that an allergy specialist can help interpret.
Someone diagnosed with celery allergy should not test the theory by cooking celery at home and deliberately eating it unless advised that this is appropriate.