An allergy is the immune system overreacting to something normally harmless, causing symptoms such as sneezing, rashes, or swelling, while an intolerance is an inability to properly digest or process a particular food or substance. A memory hook: lactose intolerance is about digestion struggling with a substance, not the immune system reacting to it.
The core difference
The core difference is what is reacting: an allergy involves the immune system overreacting, while an intolerance is a difficulty digesting or processing something.
- allergy — the immune system overreacting to a normally harmless substance: His allergy to pollen gets worse every spring.
- intolerance — an inability to properly digest or process a particular food or substance: He was diagnosed with lactose intolerance as a child.
- allergy — also used figuratively for a strong dislike: He has a real allergy to paperwork.
How to tell them apart
The word allergy was coined in 1906 by the Austrian doctor Clemens von Pirquet, combining Greek allos (other) and ergon (work, reaction) to describe the immune system reacting abnormally to something. That immune-system involvement is what separates an allergy from an intolerance.
Intolerance, by contrast, is a broader word — it comes from Latin intolerantia, not accepting or bearing something — and in medicine it describes the body struggling to digest or process a substance, such as lactose, rather than the immune system mounting a reaction to it.
