Answer the following question given this paragraph: 

An allergy occurs when the immune system attacks a harmless substance that enters the body from the outside. A substance that causes an allergy is called an allergen. It is the immune system, not the allergen, that causes the symptoms of an allergy.


Q: What occurs when the immune system attacks a harmless substance that enters the body from the outside?

 Choices:

- panic attack

- nausea

- plague

- allergy

A:
allergy
