How does a true allergy to onions or garlic trigger symptoms differently than fructan intolerance?

The immune system mistakenly identifies food proteins as a threat, releasing chemicals like histamine.

A genuine food allergy to alliums involves a fundamentally different physiological mechanism than simple intolerance. In a true allergy, the body's immune system incorrectly perceives the proteins present in the onion or garlic as a threat. This mistaken identification prompts an allergic cascade characterized by the rapid release of various chemicals, most notably histamine. This response can be immediate and severe, manifesting as hives, swelling of the mouth or throat, vomiting, or potentially life-threatening anaphylaxis. Crucially, because this reaction is IgE-mediated, it is less dependent on the consumed quantity; even trace amounts have the potential to trigger a serious response in a highly sensitized person, unlike fructan intolerance which is dose-dependent.