Cross allergy
If you are allergic to birch pollen, you may also
react allergically to eating hazelnuts and apples. This may seem
curious but it is one of a great many examples of what is known as
cross allergy. An allergy is called a cross
allergy if the antibodies that the body produces against one
allergen start reacting to another allergen as well, for the simple
reason that they are alike. They are related to each other, family,
so to speak. Appearance does not enter into it: hazelnuts, apples
and birch trees do not look like each other, but they contain
related allergens, as do kiwi, pineapple and potato, for
instance.

