When a weed and a jar of mustard become connected
Mugwort is an unremarkable-looking weed that grows widely across Europe. Its pollen is also capable of causing hay fever, particularly later in the summer.
At first glance, there is no obvious reason why somebody's reaction to pollen floating through the air should have anything to do with mustard on their dinner plate.
The connection becomes clearer when we remember that the immune system doesn't read food labels.
It reads molecules.
Proteins in apparently unrelated plants can contain sufficiently similar structures that IgE antibodies originally produced against one of them also recognise another. This is cross-reactivity.
In a condition known as mugwort–mustard allergy syndrome, people sensitised to mugwort pollen can also react to mustard and, in some cases, other plant foods.
This isn't simply a theoretical association invented because mugwort and mustard happen to be plants. Researchers investigating confirmed mustard allergy have demonstrated a strong association with mugwort sensitisation in particular groups of patients, as well as immunological evidence of cross-reactivity.
There are several families of plant proteins capable of contributing to these complicated relationships. As we saw earlier, mustard contains an nsLTP allergen, Sin a 3, and a profilin allergen, Sin a 4. Relatives of these proteins occur elsewhere throughout the plant kingdom.
This creates one of the most counter-intuitive facts about allergy:
Two foods can be botanically unrelated yet allergically connected, while two foods that are close botanical relatives may both be perfectly tolerated by the same person.
The immune system is responding to proteins, not to the categories humans use in cookery.
That is why somebody should not construct an enormous exclusion diet simply by looking up everything related to mustard on a botanical family tree.
Mustard, broccoli, cabbage and Brussels sprouts: should they all be avoided?
This is an obvious concern once somebody learns that mustard belongs to the Brassicaceae family.
The family includes foods such as broccoli, cauliflower, cabbage, Brussels sprouts, turnip and rapeseed.
So does mustard allergy mean no more broccoli?
Not automatically.
Cross-reactivity has been demonstrated between mustard and other members of its botanical family, and Anaphylaxis UK specifically advises people with mustard allergy to be aware of seeds and sprouted seeds from other Brassica-family plants because similar seed proteins may cause reactions.
But that is very different from saying that everybody with mustard allergy must stop eating every Brassica vegetable.
The part of the plant matters.
Remember Sin a 1 and Sin a 2: important mustard allergens that are seed-storage proteins. A seed is biologically different from the mature leaves, stems or flower structures we may eat as vegetables.
This is a useful reminder of how crude the word "food" can be in allergy science.
We might regard mustard seed and broccoli as two foods.
A plant scientist sees different species and different tissues.
An allergist may be interested in the particular proteins present in those tissues.
And the immune system encounters the proteins themselves.
If somebody diagnosed with mustard allergy has been happily eating broccoli, cabbage or cauliflower without symptoms, the fact that those vegetables are relatives of mustard is not, by itself, a reason to remove them.
Unnecessary food avoidance has consequences of its own: nutritional restriction, inconvenience, anxiety and a progressively shrinking list of foods considered "safe".
Questions about suspected reactions to related seeds or vegetables therefore belong with an allergy specialist rather than being answered by botanical guesswork.
Cross-reactivity can travel much further than the cabbage patch
The story becomes more surprising still.
One of mustard's important allergens, Sin a 2, belongs to a class of seed-storage proteins called 11S globulins.
Related 11S globulins occur in other seeds and nuts.
Laboratory research involving mustard-allergic patients has demonstrated IgE cross-reactivity between Sin a 2 and homologous proteins from several tree nuts and peanut.
Again, this needs careful interpretation.
It does not mean:
Mustard allergy = peanut allergy.
Nor does it mean that somebody newly diagnosed with mustard allergy should immediately remove nuts and peanuts from their diet.
It means that proteins from different plants can share structures capable of being recognised by the same IgE antibodies.
This distinction between cross-reactivity in the laboratory and a reaction when somebody actually eats a food is one of the recurring themes of modern allergy medicine.
It also explains why component-level allergy testing can sometimes provide more useful information than simply knowing that somebody has IgE against a whole-food extract.
Instead of asking only:
"Does this person's blood contain antibodies that bind mustard?"
modern allergy investigation can sometimes ask the more precise question:
"Which mustard protein are those antibodies recognising?"
That can matter because different protein families have different characteristics — including how resistant they are to heat and digestion and what other allergens they resemble.
Allergy diagnosis is gradually moving from recognising foods towards understanding molecular patterns of allergy.
That is a much richer picture.
Pollen geography can matter too
Remember mugwort–mustard allergy syndrome.
Mustard allergy is not always simply a story beginning with somebody eating mustard seed.
Cross-reactive pollen sensitisation can contribute to particular patterns of plant-food allergy.
The distribution of pollens varies geographically.
Climate influences pollen seasons.
Vegetation differs.
Individual exposure differs between urban and rural environments and between countries.
So the allergic geography of Europe partly reflects the botanical geography of Europe.
A person's immune system does not live in isolation from the environment around them.
It spends a lifetime sampling it.