Allergy (noun)

1. An abnormal reaction of the body to a previously encountered allergen, which is typically harmless, such as pollen, certain foods, or certain medications.
2. Sensitivity to a substance, typically a particular food or drug, that results in an overreaction of the immune system.

Origin:

Early 20th century: from german allergie, from greek allos "other" + -ergos "working".

Examples:

1. He has a severe allergy to peanuts.
2. She has an allergy to dust.
3. He has an allergy to the medicine.
4. She has an allergy to the pollen.
5. He has an allergy to the cat hair.