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Allergens are any of the thousands of food, chemical, plant or animal products that cause a specific allergic reaction in individuals. Because of the vast number of things that might prompt the immune system to think it is under attack by a foreign substance, allergens can be a little tricky to identify in some cases. For example, during the hay fever season, you might suffer from symptoms but be unsure as to what particular pollen it is that is causing the allergic response. In food, allergens can be anything from an obvious allergy to shellfish or peanuts, or due to a more complex allergen involving the chemical makeup of different components in a particular food family. A good example of this is the nut family. Because of the different arrangement of proteins in different nuts (which will determine their shape and flavour) it’s possible to be highly allergic to an allergen produced in a walnut, whereas eating brazil nuts cause no allergic reaction at all.