Cross allergy

and Martina Feichter, medical editor and biologist and Carola Felchner, science journalist

Mareike Müller is a freelance writer in the medical department and assistant doctor for neurosurgery in Düsseldorf. She studied human medicine in Magdeburg and gained a lot of practical medical experience during her stays abroad on four different continents.

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Martina Feichter studied biology with an elective subject pharmacy in Innsbruck and also immersed herself in the world of medicinal plants. From there it was not far to other medical topics that still captivate her to this day. She trained as a journalist at the Axel Springer Academy in Hamburg and has been working for since 2007 - first as an editor and since 2012 as a freelance writer.

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Carola Felchner is a freelance writer in the medical department and a certified training and nutrition advisor. She worked for various specialist magazines and online portals before becoming a freelance journalist in 2015. Before starting her internship, she studied translation and interpreting in Kempten and Munich.

More about the experts All content is checked by medical journalists.

In the case of a cross allergy, the immune system is over-sensitive to the same or similar allergenic substances (allergens) from different allergen sources: First of all, the person affected develops an initial allergy, for example to birch pollen. An apple or hazelnut allergy, for example, can occur later as a cross-reaction because these foods contain proteins that are similar to those in birch pollen. Read more about cross allergies here: development, symptoms, treatment and prevention.

ICD codes for this disease: ICD codes are internationally recognized codes for medical diagnoses. They can be found, for example, in doctor's letters or on certificates of incapacity for work. L23J30

Cross allergy: definition

Sometimes people with an allergy to pollen, latex or house dust mite droppings suddenly become over-sensitive to certain plant or animal substances - mostly food. Doctors then speak of cross allergy:

Every allergy is an overreaction of the immune system against an actually harmless substance (called an allergen). Such allergens are often vegetable or animal proteins. The immune system of allergy sufferers mistakenly regards the allergens as dangerous and fights them - an allergic reaction results, i.e. an excessive defense reaction.

For example, some people have a birch allergy, more precisely: an allergy to certain proteins in the birch pollen. It can then happen that the excessive immune response occurs suddenly even after eating certain foods such as apples, carrots or celery. The reason: These foods contain proteins that are structurally similar to the allergens in birch pollen - a cross-reaction occurs due to structurally similar allergens.

Some foods are more likely to be cross-allergic than others, such as hazelnut, peanut, and celery.

Often the symptoms of a cross allergy are only mild. In some cases, however, severe cross-reactions that affect the entire body also occur.

In Germany there are around 3.5 million food allergy sufferers in the age group of 15 years and over (adolescents and adults). In around 60 percent of them - i.e. more than two million people - the food allergy is the result of a cross-reaction. The frequency of cross allergies is tending to increase in Germany.

Cross allergy: forms

Which substances can trigger a cross-reaction in individual cases depends, among other things, on the primary allergy trigger. Some examples:

In general, allergies to tree pollen often lead to stone fruit allergies (e.g. peach, nectarine, apricot allergy) as a cross reaction - as well as to nut allergies, for example to a cross allergy with hazelnut (hazelnut allergy).
Hazelnut, celery and / or stone fruit often cause a cross allergy in those people who are allergic to birch (i.e. birch pollen). A strawberry allergy is also possible as a cross-reaction here.
With an allergy to grass pollen, those affected often cannot tolerate peanuts and legumes (such as peas, beans), develop a tomato allergy or an allergy to cereals (e.g. wheat, rye allergy).
Cross allergies in rye pollen allergy sufferers can in turn affect grass and grain varieties such as barley, oats, maize, wheat or rapeseed.
In general, a pollen allergy often leads to a cross-reaction to an allergy to plant-based foods (e.g. a fruit allergy).
A celery allergy can develop as a cross-reaction based on a mugwort allergy. Some mugwort allergy sufferers are also hypersensitive to potatoes or certain spices.
Hypersensitivity to shrimp can develop as a cross allergy in house dust allergy sufferers.

In addition to these well-known forms of cross allergy, new variants are constantly being added as a result of changing eating habits. For example, a reaction to the "functional food" acerola cherry has already been observed in people with a latex allergy. Dragon fruit and pomegranate allergies have also already been documented.

Sound confusing? The following cross-allergy table gives you an overview of important cross-reacting allergens.

primary allergy	possible cross-reactions
Birch pollen	Cross allergy to apple, carrot, soy, celery, hazelnut, kiwi, strawberry, fresh stone fruit, etc. "Birch pollen-nut-pome fruit syndrome"
Grass pollen	Cross allergy to peanuts, soy and other legumes, cross allergy to rye, wheat or other cereal flour, etc.
Mugwort pollen	Cross allergy to celery, carrots, potatoes, spices (anise, curry, cinnamon) etc. "Celery-Carrot-Mugwort-Spice Syndrome"
latex	Cross allergy to avocado, banana, kiwi, etc. "Latex fruit syndrome"
Dust mite droppings	Cross-allergy to shellfish / crustaceans
Bird feathers	Cross allergy to chicken eggs (egg yolk) and poultry meat "Bird and Egg Syndrome"
Cat hair	Cross allergy to pork "Cat Pork Syndrome"
Cow's milk	Cross allergy to beef

Reverse cross-reactions than those indicated above are also possible. Anyone who is allergic to kiwi, for example, can suddenly show a cross allergy to latex.

A cross allergy cannot only relate to food. For example, as a result of a latex allergy, cross-reactions with various ornamental and indoor plants such as oleander, poinsettia, rubber tree and / or weeping fig (Ficus benjamini) can occur.

Cross allergy: symptoms

The symptoms of cross allergy usually appear a few minutes to two hours after eating. They are often mild and limited to a certain area of the body.

In adults with a cross allergy, what is known as urticaria (hives) usually develops in the oral mucosa after or when eating. This is a collection of wheals on the usually reddened mucous / skin. The lining of the mouth burns and itches, the tongue tingles. Physicians also refer to these symptoms as "oral allergy syndrome".

In addition, other complaints in the mouth area can occur:

Itching as an allergic reaction on the lips, tongue, roof of the mouth and / or throat
Swelling of the lining of the mouth
Reddening of the skin on the lining of the mouth and around the mouth
Blistering, especially on the lips

In rare cases, the cross allergy can affect the entire body. The most common are skin symptoms such as:

Wheals all over the body with itching (also a common symptom of contact allergies, e.g. with detergent or nickel)
sudden severe and "pasty" swelling of the skin (angioedema)
Flush symptoms: sudden severe reddening of the skin, sometimes accompanied by itching
Worsening of existing eczema in patients with atopic dermatitis

Under certain circumstances, more extensive symptoms can occur in the case of a cross allergy:

Gastrointestinal complaints such as vomiting or diarrhea
Shortness of breath
Circulatory problems such as dizziness and a drop in blood pressure

If someone develops a rash, shortness of breath, and cardiovascular problems, it is most likely anaphylactic shock. Then you need to quickly provide first aid and alert the emergency doctor!

In addition, the allergic reaction to cross-reactive foods is often milder after the pollen season from around March to the end of August.

Cross allergy: treatment

The treatment of a cross allergy usually depends on its severity and the underlying primary allergy. In any case, people with cross allergies should avoid known allergens if possible. The symptoms of cross allergy can be relieved with medication. If necessary, one can try to treat the underlying allergy by means of desensitization. As a result, the cross allergy may also disappear.

Medication

To treat the symptoms of cross allergy, various medications are available in tablet form, inhalation sprays, ointments or injections.

Antihistamines such as cetirizine block the docking points (receptors) of the messenger substance histamine and thus its effect - histamine is significantly involved in mediating allergic reactions.

Glucocorticoids ("cortisone") such as prednisolone inhibit the immune system. This can limit or even prevent an allergic reaction.Prednisolone is given along with antihistamines and adrenaline, for example in anaphylactic shock.

So-called mast cell stabilizers such as cromoglycine prevent histamine from being released from mast cells. This can contain the allergic reaction.

Avoid allergens

If you have been shown to have a cross allergy, you should avoid the affected food or prepare it differently. Most proteins lose their allergenic effect in high heat. If, for example, raw carrots cause oral allergy syndrome in you, you can try it cooked - you may be able to tolerate them better this way.

If you are allergic to several foods and have to avoid them, you should take special care to eat a balanced diet in spite of everything. Your doctor or nutritionist can help you devise an appropriate nutrition plan. Also keep in mind: If a pollen allergy results in a cross-reactive reaction to certain foods, this reaction is often milder outside the pollen season.

It is not advisable to delete suspicious foods from the menu if you are lucky (elimination diet). You may risk a shortage of supply. Always discuss an elimination diet with an expert, such as a nutritionist, first.

Possibly desensitization

Desensitization is a lengthy treatment in which the doctor administers the allergen to the allergy sufferer in slowly increasing doses so that the immune system slowly "gets used" to it. Ideally, this treatment can relieve the person affected from the primary allergy (e.g. birch pollen allergy). Under certain circumstances, the cross allergy also disappears.

However, this does not always work, and the result for the individual case cannot be estimated beforehand. This was the result of a study with pollen allergy sufferers who showed cross-reactions with food. According to the researchers involved, desensitization should therefore not primarily serve to treat the symptoms of a cross allergy - the chances of success are too uncertain. Instead, desensitization should only be carried out if the symptoms of the underlying primary allergy speak in favor of such a treatment.

Cross allergy: you can do that yourself

Many cross-allergens are not stable to heat, acid, or exposure to oxygen. So it can help to prepare cross-reactive foods appropriately. Apples or cherries in cake form (i.e. baked) are often well tolerated. If you have a cross allergy to tomatoes, it can help to heat them up (e.g. to dry them in the oven).
Not all apples are the same. The allergen content varies depending on the variety. Gloster and Altländer, for example, often cause no or fewer symptoms than other varieties in the case of a cross allergy with birch. Scientists also found a difference in tomatoes. With cross allergies, varieties such as Rugantino or Rhianna are better tolerated, as they contain the least of the allergy-causing protein (Sola I 4).
Don't mix. Sometimes it is the case that allergy sufferers can tolerate a type of fruit or vegetable, but not a fruit salad or a raw vegetable plate. Avoid such a cumulative effect.
Stress and physical exertion can lower the response threshold. You should therefore not eat any foods that are suspicious of allergies during turbulent phases in life or before and after exercise.
A cross allergy can lead to life-threatening circulatory problems. You should therefore be well informed about your illness and know what to do in the event of an allergic reaction. Always carry your allergy pass and an allergy emergency kit with medication with you.

Cross allergy: causes and risk factors

Every cross allergy is based on a "primary" allergy: At first there is an over-sensitivity (allergy) to pollen, house dust mites or latex - more precisely: to certain proteins that are contained in them. They are called allergy triggers (allergens). The immune system falsely classifies these proteins as dangerous upon first contact and produces specific antibodies against them (immunoglobulin E). The next time they come into contact, these antibodies bind to suitable points on the surface of the proteins in order to render them harmless. It sets in motion an immunological defense reaction against the supposedly dangerous intruder, which is reflected in allergic symptoms (mediated by the messenger substance histamine).

With cross allergies, the immune system no longer only reacts to the primary allergens - such as the proteins in birch pollen - but also to other proteins that are structurally similar, for example those in apples, carrots, celery or hazelnuts. To a certain extent, there is a mix-up: the antibodies that specialize in birch allergens also attach to the similar allergens in apples, carrots, etc., which then also leads to allergy symptoms.

Risk factors

Anyone can develop an allergy and thus a cross allergy in the course of their life. However, there are some factors that increase the risk of allergies. These include:

genetic predisposition
High-fat diet, fast food, food contaminated with colorings and preservatives
smoking
Air pollution
excessive hygiene
no breastfeeding in infancy

Cross allergy: examinations and diagnosis

If someone is allergic to something, it must be clarified whether it is a "primary" allergy or a cross-allergy - a cross-reaction to which there is a primary allergy trigger. For example, a supposed walnut allergy can be a cross allergy caused by a tree pollen allergy, an allergic reaction to potatoes a cross allergy to mugwort, and a peanut allergy sometimes turns out to be a cross allergy with grass pollen as the primary allergy trigger.

anamnese

If a cross allergy is suspected, the doctor first collects the patient's medical history (anamnesis) in conversation with the patient. Possible questions are:

Do you have any known allergies? If so, which ones (e.g. hay fever)?
Do you suffer from neurodermatitis or allergic asthma?
When exactly did your current allergic symptoms occur?
What are these symptoms (swelling of the oral mucosa, rash, etc.)?
Did you eat or drink something shortly beforehand?

Testing

The doctor then carries out an allergy skin test - more precisely a prick test. To do this, he drips various allergen solutions (e.g. individually on the patient's forearm or back. Then he pricks the skin lightly at the relevant points with a needle or lancet, so that the skin barrier is damaged under each drop. If the patient hits an or If you are allergic to several of the test allergens, local hypersensitivity reactions such as reddening of the skin develop in the affected areas after a short time.The prick test is then positive for the allergen in question.

As an alternative to the prick test, an antibody determination is also possible. To do this, the doctor takes a blood sample and has it checked for cross-reactive antibodies.

The oral provocation test can be carried out if the prick test gives an unclear result. The patient ingests the suspicious food under medical supervision. Then it is observed how the body reacts to it. If possible, the provocation test should be placebo-controlled and double-blind: Neither the doctor nor the patient know whether the latter has just swallowed a dose of the food or the allergens it contains or an allergen-free comparator preparation (placebo).

Cross allergy or pseudo allergy? The differences

So-called pseudo-allergies to food such as histamine intolerance or intolerance to certain food additives (e.g. E numbers) must be differentiated from a cross allergy to certain foods. Because pseudo-allergies are not an allergic reaction in the strict sense, even if their symptoms are very similar to those of a cross-allergy to food. But there are differences:

The symptoms of a pseudo-allergy appear a little later after eating and last longer.
The prick test and the IgE antibody determination are negative in the case of a pseudo-allergy.
A desensitization is ineffective in pseudo-allergies.

Cross allergy: disease course and prognosis

The severity of a cross allergy varies. There are patients with a pronounced pollen allergy who only show a slight cross-reaction to a certain food. However, it can also be the case that someone only has a very slight pollen allergy but develops severe allergy symptoms after consuming a food that is cross-reactive with it.

In addition, a cross allergy can get worse or affect more and more foods over the years. But it can also go away on its own after a while.

The severity of a cross-reaction can also vary - for example, depending on the season and pollen count (in the case of underlying hay fever), environmental toxins to which the person concerned is exposed, and the psychological state of the person concerned.

Cross allergy: prevention

In general, it is important to treat the causal allergy. Why allergies develop has not yet been fully clarified, but the risk factors listed above are considered to be certain.

If you have a cross allergy, you should avoid the affected foods or prepare them differently. Other foods that can potentially trigger cross-reactions should be consumed with caution. For example, if you suffer from a latex allergy and show symptoms of an oral allergy syndrome after consuming a kiwi, it is advisable not to consume fresh avocados either.

A healthy lifestyle and physical activity strengthen the immune system. This can reduce the symptoms of allergies and possibly even prevent the occurrence of a cross allergy.

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