extreme sensitivity, marked by sneezing, itching, skin rashes, etc, to substances that have no such effect on the average individual
Allergies are caused by extreme sensitivity (hypersensitivity) of the immune system to innocuous substances, known as ‘allergens’, which do not usually trigger an immune response in a normal healthy individual. As part of the immune response, the affected tissues release histamine and other chemicals, which results in symptoms such as swelling, itching, constriction of the airways and rashes. A life-threatening allergic reaction is known as anaphylaxis — Dr John Cormack
increased sensitivity to an antigen in response to a first exposure
informal a feeling of antipathy or aversion
[German Allergie, from Greek allos other + ergon work]