Secondary food allergies usually appear in older children, adolescents and adults who have a primary allergy to airborne substances such as pollen (medical term: aeroallergens). This type of allergy is often referred to as oral allergy syndrome or cross-reactivity. Cross-reactivity to food also occurs with latex allergies. Since certain proteins in fruits or vegetables are similar to pollen and latex allergens, allergic individuals can also experience an allergic reaction when they eat these foods.
The prevalence of cross-reactions to foods is not precisely known and can therefore only be estimated. Based on interviews with doctors, scientists at the Robert Koch Institute assume that a total of 4.7% of all adults in Germany have some form of food allergy. Allergy organisations estimate that up to 60% of the food allergies among older children, adolescents and adults are cross-reactions related to a primary sensitisation to aeroallergens. The severity of the primary allergy is irrelevant in a cross-reaction. Sometimes the primary allergy itself only triggers mild symptoms or none at all; in this case, the allergic individual only experiences symptoms from the cross-reaction to the food. Cross-reactivity can also develop long after the primary allergy.
Triggers include pollen in particular, but also house dust mites and animal hair. The birch allergen Bet v 1 often leads to cross-reactions to hazelnuts and other nuts, apples, drupes and other stone fruits, carrots and soya, while people who are allergic to mugwort or ragweed often experience allergic reactions to celery, spices, cucumbers, melons or bananas. It is important to note that cooked or baked fruit can usually be tolerated since the allergen is destroyed by heat.
From a biological standpoint, there is an interesting link between house dust mite allergy and crustacean and mollusc allergy. Since house dust mites, like crustaceans, belong to the phylum of arthropods and likewise contain a certain type of protein, some people who are allergic to house dusts mites develop cross-reactivity to lobster, crab and other seafood.
Overview of cross-reactions between aeroallergens and foods
|Respiratory allergy||Food allergy|
|Common cross-reactions||Tree pollen (e.g. birch, hazel)||Apple, peach, plum, nectarine, kiwi, cherry, pear, almond, hazelnut and other nuts, carrot, celery, potato (raw), soya, lychee|
|Ragweed||Melon, banana, tomato, cucumber|
|Mugwort||Carrot, celery, cumin, parsley, coriander, anise, fennel seed, mango, grape, lychee, sunflower seed|
|Latex||Banana, avocado, potato, tomato, kiwi, pineapple, chestnut, buckwheat flour, celery, fig|
|Rare cross-reactions||House dust mite||Prawn, crab, lobster, shrimp, snail|
|Grass and grain pollens (e.g. wheat, rye)||Spelt, barley, oat, millet, corn, wheat, rye (including flours and brans made from them), tomato, legumes|
|Ash pollen/olive pollen||Pineapple, horseradish|
|Bird allergens (feathers, droppings)||Egg, poultry, organ meats|
|Animal hair||Cow’s milk, meat and organ meats|
Secondary food allergies usually trigger only mild, local symptoms, but severe allergic reactions are theoretically possible – especially if the allergic individual ingests a large amount of the allergenic food. Symptoms occur a few minutes to two hours after the food is eaten and most commonly affect the oral mucosa or the skin and, in some cases, the breathing. Gastrointestinal and cardiovascular symptoms can also occur, but these are not as common and usually accompany other symptoms.
The most common reaction is known as oral allergy syndrome or contact urticaria of the oral mucosa, in which the patient experiences itching of the lips, tongue, gums, ears and throat. Sometimes the mucosa swells, or redness and small blisters appear. These symptoms usually subside quickly.
Cross-reactions can also affect the skin with symptoms such as swelling, itching or redness.
Food allergies due to cross-reactivity in childhood and adulthood
Medical experts currently believe that primary food allergies mostly appear in childhood when the digestive tract comes into contact with the allergen, while cross-reactivity often does not appear until later in life. This occurs when the allergic individual develops an allergy by breathing in airborne substances and, as a result, also reacts to related proteins in food.
It is theoretically also possible for children to develop a secondary, pollen-related food allergy. It is also possible for people in late adulthood to spontaneously develop a primary food allergy. However, is currently no scientific evidence to support this.
For the diagnosis, the doctor first asks the patient about his or her eating habits and related symptoms (medical term: anamnesis). If the trigger is unknown, the patient can keep a food and symptom diary, which helps the doctor determine the causes. The initial suspicion is then further investigated by conducting a skin prick test or a blood test. An oral provocation test is only performed in rare cases in which the cause remains unclear and a severe reaction has already occurred. For this test, the patient gradually ingests small amounts of the suspected allergen under supervision. However, due to the risk of a severe reaction, this test is only performed under medical supervision and often requires a hospital stay.
The most reliable way to prevent cross-reactions to food is to abstain from eating the food in question (allergen avoidance). Some individuals only need to avoid the food during the pollen season or to prepare it differently (e.g. cooked instead of raw) because some allergens cannot withstand high temperatures. It is not recommended to avoid certain foods without a confirmed diagnosis. Furthermore, any changes to the diet should be supervised by a qualified nutritionist in order to ensure that no deficiencies occur.
If patients experience severe allergic reactions, they should carry emergency medications with them. An emergency kit for these reactions contains an adrenaline auto-injector, a corticosteroid (in liquid or tablet form) and an antihistamine (in liquid or tablet form).
Although there are now indications that a secondary food allergy can be improved through specific immunotherapy against the primary allergen (such as pollen), this treatment should only be used when the symptoms of the primary allergy are the priority.
- In many cases, cooked fruits or vegetables are tolerated because the allergy-triggering proteins are destroyed by heat. Peeling can also increase tolerability.
- Some types of fruit are better tolerated than others. This holds particularly true for apples. An overview of the better tolerated apple varieties is available from BUND Lemgo (the Lemgo local environmental protection association).
- Be aware that stress, alcohol consumption or the use of certain medications can increase the severity of an allergic reaction.
- Pollen-related foods may not be as well tolerated during pollen season than during periods with a low pollen count.
Prof. Dr. med. Dr. h.c. T. Zuberbier
Last update: November 2016