5 key food allergens have been identified in barley, Hor v 12, Hor v 15, 16, 17 and Hor v 20.
Additionally Hor v 5 is an allergen associated with airway allergies, so is important for those sensitised to barley pollen.
Hor v 20 is a gamma hordein and is the allergen most commonly linked to cross reactivity with other grains.
As barley contains a gamma hordein it is commonly linked to exercise induced anaphylaxis, more specifically to wheat dependent exercise induced anaphylaxis. This is because the gamma hordein is similar in structure to omega-5 gliadin's found in wheat.
Coeliac disease is not an allergic condition, but is triggered by gluten found in wheat and there are similarly shaped proteins in barley to which sensitive individuals may react.
Barley is in the Poales order of foods, so there may be some cross reactivity with other foods in the group such as wheat, rye and maize.