hypersensitivity reaction to a particular allergen; symptoms can vary greatly in intensity
"After eating strawberries, she experienced an allergy that ranged from mild itching to severe swelling of her lips."
A disorder of the immune system causing adverse reactions to substances (allergens) not harmful to most and marked by the body's production of histamines and associated with atopy, anaphylaxis, and asthma.
"Her severe allergy triggered a cascade of histamine release after she touched pollen, leading to swelling and difficulty breathing typical of her atopic condition."
In plain English: An allergy is when your body has an overly strong reaction to something harmless that most people can handle without any trouble.
"She has an allergy to peanuts, so she always checks food ingredients carefully before eating."
Usage: Use allergy as an uncountable noun when referring to the condition in general, but add a specific allergen after it if you need to identify what triggers the reaction. Avoid confusing this medical term with intolerance, which involves digestive issues rather than an immune system response.
The word allergy entered English from German, where Austrian pediatrician Clemens von Pirquet coined it in 1906 to describe a reaction involving "other work." He formed the term by combining Ancient Greek roots for "other" and "activity," following the pattern of the word energy.