Allergies are caused by reactions of the body’s immune system to a wide variety of sources. These include animal products, mites, moulds, latex, pollen, medicines, plants, venom as well as food.

Individual types of allergy
Allergies often involve multiple organs, e.g. gastrointestinal and respiratory systems including the sinuses, the eyes, skin and even the nervous system. Careful observation and history taking by the physician are essential to discover patterns of associations between allergies.

Common cross-reactions between pollen and food
Birch pollen: apple, carrot, celery, pear, tomato, cherry, tree nuts (infrequently: many more fruit and spices)
Goosefoot pollen: banana, melon, peach (infrequently: nectarine, asparagus, kiwi, potato, olive, onion)
Mugwort pollen (weed): carrot, celery, aniseed, peach (infrequently: many more fruit, vegetables and spices)
Ragweed pollen: melon, cucumber, banana, sunflower
Timothy grass: apple, litchi, tomato, celery, corn, bell pepper, paprika

Most individuals with hay fever cross-react with two or more foods. The most frequent symptoms are tingling, swelling, itching in the mouth or on the lips, but about 10% have intestinal symptoms.

Common cross-reactions between different foods
Shrimp, lobster, crab, less common: squid, scallops, clams, oysters (associated: mite allergy)
Squid, scallops, clams, oysters, less common: shrimp, lobster, crab,
Peanut, lupine
Tree nut (walnut, hazelnut, Brazil nut, pecan), pistachio, cashew
Cow’s milk, all mammal’s milk (e.g. >90% with goat and sheep milk, beef meat (10%!))
Hen’s egg, rarely other bird’s eggs and poultry meat
Cereal (wheat, oat, barley, rye, millet, sorghum, maize, rice): cross-allergy not common. 20% cross-react with one other cereal.
Flour allergy – consider mite contamination.
Fish, frequent cross-reactions, even between salt and fresh water fish. Not to shellfish. Distinguish from histamine intolerance (i.e. non-allergic, see Biogenic amines)
Seeds, sesame, mustard, sunflower
Peach and melon commonly cross-react with other fruit (generally mild reactions)

Latex- fruit allergy syndrome –latex contact allergy associated with food allergy in 30-80% of cases
Latex, bananas, avocado, chestnut, apple, kiwi, potato, tomato, melon, papaya. Less common: fig, pineapple, peach, pear, passion fruit, walnut, hazelnut, almond, grapefruit, strawberry, spinach, lettuce, celery, diverse spices. Contact allergy with latex products, as well as certain plants (e.g. fig, gum, oleander, cactus).