Plant cross reactivity and its fallout for allergy sufferers

Sue Killian is an environmental biologist with a very personal interest in allergy. She describes how so many of our plants are inter-related, not only with each other but with many of the foods that we eat. Her book Allergy and Cross Reactivity is now available on Amazon here.


Allergens are molecules, not whole plants or animals. An allergen molecule found in one pollen can be hiding another kind of pollen or even in a food. Therefore, it is said that pollen allergens and food allergens can cross-react. There is much value in limiting pollen exposure to avoid future food allergies, especially in children, for their allergies will change throughout their lifetimes. Their pollen sensitivity of today may become their food allergy of tomorrow.         

There are allergenic molecules that only occur in some plants and these are called species-specific allergens. For example the first allergen discovered in ragweed seems to be species-specific for it has only been found in ragweed, not in other plants. These species-specific allergens are not cross-reactive molecules. But included in the ragweed, and birch pollen packages are other allergenic molecules and some of these do appear in other species. The similar molecules which appear in more than one species are the cross-reactive molecules. 

Individual reactivity

Because there are several allergens in one plant, patients can react to these assorted allergen molecules of the same plant, such as ragweed, in different ways. Each patient has his/her own personal reactivity pattern. There are at least ten allergens in ragweed pollen and seven in birch tree pollen. (1) So, you could be allergic to anything from two to seventeen allergens if you were allergic to both of these allergen packages. You might be unlucky enough to react to all of these allergens, or some of them, or only one allergen in ragweed, and one allergen in birch pollen.

Many plants and animals have allergens which are not specific to that particular source. Birch pollen allergens are found in apples and latex allergens are found in bananas. (2,3) Grass pollen allergens are found in wheat. (4) The observation that some plants, pollens, and even animals share allergens has led to a whole new study, the cross-reactivity of allergens. The allergens which are found in more than one plant or food sensitize patients and cross-react according to the patient's exposure to them.

'A web of continuous reactivity'

Cross-reactive allergens can affect a patient negatively by keeping the patient caught in a web of continuous reactivity. Even when patients try to avoid a specific allergen, that allergen can be hiding in another food or pollen.            

It is good to be mindful of cross-reactive allergens when you are planning your diet, your landscaping, your garden, and when you are planning immunotherapy. Therefore, it is useful for you, as a patient, and for your physician, to be aware of cross-reactivity.


The fact that there can be many allergens in one plant necessitates a new system of naming allergens. In the Linnaean System each plant and animal has a Latin genus name followed by a species name.  Now that it is known that allergens are specific molecules within a plant or animal, the Linnaean System remains useful but an additional layer of naming distinguishes the biochemical properties. Allergens are named using the first three letters of the Latin genus, followed by a single letter for the species and a number showing the order in which each allergen was purified. For example, the first antigen purified for white oak, Quercus alba, is Que a 1. (5)

The plant world offers unlimited opportunities for the study of cross-reacting allergens. These may be pollens of trees cross-reacting with other trees, weeds, grasses or foods. Weed pollens can also initiate cross-reactive reactions or be the target of cross-reactions, as can grasses or foods.

BBQTree Pollen Cross-reactivity


Tree to tree cross-reactivity is a good starting point to study cross-reactive pollens. Birch (Betula verrucosa), its Bet v 1 and Bet v 2 allergens, were the subjects of early cross-reactivity studies. These birch allergens were found to cross-react with alder (Alnus glutinosa, Aln g 1), hornbeam (Carpinus betulus, Car b 1), hazel (Corylus avellana, Cor a 1), chestnut (Castanea sativa, Cas s 1) and oak (Quercus alba, Que a 1) pollens. (6) It was then found that birch pollen alone was sufficient to desensitize patients to all of these tree pollens. (7)  In Southern Italy, scientists have found that food sensitive, pollen allergic patients are sensitized directly to oak and hornbeam without the influence of birch. (8,9) This is an example of geography influencing a patient's individual reactivity pattern.

Swedish scientists who studied birch tree pollen cross-reactivity observed that cross-reactivity occurred not only between trees but also between trees and foods. Specific food allergies were observed to occur during tree pollen allergy season, leading to a realization that tree pollen and food allergies were related. Doctors noticed patients with springtime hay fever complained of itching and tingling of the lips, tongue and throat after eating hazelnuts during birch pollen season. (10) The cluster of foods to which birch pollen sensitive patients reacted were termed "birch pollen related foods" (BPRF). (11)                      

Birch Pollen-Related Foods and Trees 

Foods Trees
apple alder
almond aspen
carrots beech
celery elm
cherry hazel
hazelnut linden
orange oak

Birch Pollen-Related Botanical Families

Betulaceae Salicaceae
birch aspen
alder sallow
Fagaceae Solonaceae
oak potato
Juglandaceae     Tiliaceae
walnut linden
Rosaceae Ulmaceae
apple elm
pear Umbellifereae
almond carrot
plum Rutaceae
cherry orange

Studies in the northern European countries firmly established that birch pollen is cross-reactive with many other tree pollens and foods. If you have pollen allergies you probably would not want to landscape with birch trees. But birch trees are a northern tree species. Patients in southern climates also need to consider tree pollens with the potential for cross-reactions.    


Another set of cross-reactive allergens finds its sensitizer in olive tree pollen (Olea europaea) which is cross-reactive with privet (Ligustrum vulgare), ash (Fraxinus spp.) as well as lilac (Syringa sp.) and forsythia (Forsythia sp.), all of which belong to the olive family. (12.13.14) However, olive cross-reacts with pine (Pinus sylvestris) and cypress (Cupressus sempervirens), members of the grass family and mugwort (Artemesia vulgari) which is not even closely related.(15) In addition to these pollens, a recently discovered olive pollen allergen (Ole e 10) cross-reacts with latex, tomato, kiwi, potato and peach. (16) Therefore, a person sensitive to olive pollen might have allergy symptoms when exposed to any or all of these otherwise unrelated plant pollens and foods as well as exhibiting a latex sensitivity.

The Latin names of the plants, a globally uniform system of abbreviations and allergen names, developed by the International Union of Immunological Societies are essential tools in our discussion of cross-reactivity. Because of this universal language of molecule identification, scientists from India, Iran and the United States or any area of the globe, can be assured they are sharing similar information.

Weed Pollen Cross-reactivity

Weeds cross-react with plant-derived foods found in at least a dozen botanical families. The extensive relationships of weed allergies to food allergies has led to some specifically named syndromes or associations:

  • The ragweed-melon-banana association includes watermelons, cantaloupes, honeydew melons, zucchinis and cucumbers as well as bananas.(17)
  • Mugwort has several named syndromes to describe its cross-reactivity with foods. The celery-mugwort-spice syndrome involves cross-reactivity between mugwort and a long list of spices in addition to carrots and celery. These "spices" are parsley, caraway seeds, fennel seeds, coriander seeds, anise seeds,(18,19,20) paprika, pepper, (21) mango, (18) garlic, onion, and leek. (22)
  • The mugwort-mustard-allergy syndrome involves cross-reactivity between mugwort and the cruciferous vegetables mustard, broccoli, cabbage, and cauliflower. (23)
  • Cross-reactivity between mugwort and peaches is the mugwort-peach association (24,25,26).
  • Cross-reactivity between mugwort and chamomile tea is the mugwort-chamomile association. (27)       

Mugwort, chamomile, and ragweed are all members of the Compositae plant family, so because of this close botanical relationship as well as mugwort and ragweed cross-reactivity, it might be wise to avoid a cup of chamomile tea during ragweed season. If you are severely ragweed-sensitive and have tried chamomile tea you probably already realize that chamomile tea is not a tea for you in any season. Better news is that effective immunotherapy for ragweed or mugwort allergens has the potential to desensitize the patient to their cross-reactive foods – even chamomile.

Three other botanical families of weeds have their cross-reactivity described as named associations.

  • Plantains, a starchy banana-like food of the banana genus Musa, cross-react with melons are described as the plantain-melon association. (28,29)
  • The weed pellitory (sticky-weed) cross-reacts with pistachio described as the pellitory-pistachio association. (30,31)
  • Goosefoot (lambs quarters) cross-reacts with peaches, bananas, melons, and garlic named as the goosefoot-fruit association. (32,33)

Grass Family Cross-reactivity

Grass family members are very crossreactive.  Cross-reactivity between oat and rice was described in 1975, (34) between corn and potato in 1987. (35)  Phl p 4 of timothy grass (Phleum pratense) cross-reacts with Amb a 1, the major allergen of ragweed. (36)

Tree Sap Cross-reactivity       

The cross-reactive allergens of the tree pollens can be found in the sap of trees. When maple sap is concentrated into maple syrup or maple sugar it can be allergenic. If you are sensitive to tree pollen, the syrups and candies from tree sap may cause allergic symptoms.

Your personal reactivity pattern comes into play with any tree allergen that is cross-reactive with foods. If you know you are sensitive to tree pollens, it is wise to be careful with syrups and other foods that are either related or cross-reactive to those trees especially when those trees are pollinating. Maple pollinates early in the spring so maple syrup on your pancakes could give you a double dose of maple allergens.

The sap of the rubber tree is familiar to us as latex and contains a notorious set of cross-reactive allergens. The rubber tree is a member of the Euphorbiaceae plant family which has a latex-like sap. If you suspect you are latex-sensitive, avoiding the sap from other members of this plant family might be a consideration when shopping for your flower garden or a new house plant which has a milky sap. Most plants have not yet be investigated for latex sensitivity.           

Allergens in Smoke 

Tree pollen is not usually eaten, but mesquite wood is used to smoke foods and it does induce allergy symptoms. Chefs have been affected by the burning of mesquite wood when inhaling its vapors. Proteins weighing 59 and 66 kD were identified in mesquite smoke as human allergens. (37) This is an interesting example of how a tree pollen can become an allergenic smoke or food. A pollen-sensitive patient might eat mesquite-barbecued chicken and mistakenly think that they are reacting to chicken, not the mesquite smoke the chicken is wearing.

Investigating the allergenic smoke of a specific burning wood is another avenue of cross-reactive interest. A tree pollen-allergic person with a wood burning stove could easily be affected differently by the burning of oak, cherry, or pine woods and the specific allergens of each as well as cross-reactive allergens in their various smokes. If you are allergic to hickory pollen or hickory smoke, would you want to eat hickory smoked bacon during tree pollen season?

Managing Exposure  

When a new food is introduced, even if it is a nutritious food or even if it has therapeutic value, the possibility that it contains allergens is real. Allergy patients gradually learn through trial and error that it is wise to introduce new foods, one at a time, and watch for a reaction. While an allergic reaction occurs only during the second and subsequent exposures to an allergen, patients might have had an unrecognized previous exposure to the cross-reactive allergens in any new food. If they were previously sensitized by a pollen, the food reaction could occur from the first food exposure.

Pollen cross-reactivity with foods is a great reason to create a low pollen environment.  Yet, people thoughtlessly plant birch trees and many other cross-reactive pollens for decorative landscaping. For those allergy sufferers fortunate enough to access immunotherapy, desensitization to one cross-reactive plant pollen, such as birch, will downgrade the food allergens that cross-react with birch.  You may find that as your pollen allergy diminishes, you can tolerate the cross-reacting foods with less difficulty.  However, all allergy sufferers do not have access to medical care for their allergies. The wise landscaper will learn about low allergen trees, for less pollen means fewer  pollen allergy reactions, and because of cross-reactivity, fewer food allergy reactions.

Sue runs an allergen support consulting agency - – do contact her there for further advice.

Sue's book Allergy and Cross Reactivity is now available on Amazon here.

For more on low allergen planting and gardening see the Safe Gardening site, Tom Ogren's AllergyFreeGardening site and Nigel Clarke's amazing low allergen planting work in Guernsey.




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