Barley contains proteins called hordeins that are structurally very similar to wheat's trigger proteins.

Because of this similarity, the immune system often cannot tell them apart — research shows that 8–60% of wheat-allergic people also react to barley when formally tested. For most adults, the main source of barley is beer, brewed from malted barley. Malt extract — derived from barley — also hides in many breakfast cereals, some soy sauces, and malted milk drinks. Barley does not cross-react with rice or corn, so these remain safe grain alternatives. Sourdough fermentation can break down some of the problematic barley proteins, similar to its effect on wheat.