Fact 1: Antigens are molecules that the immune system recognizes as foreign to the body.

Fact 2: An allergy is a reaction to a substance the body recognizes as foreign.

Given the two facts above, answer the question "An allergy can be caused by" with the following options: 
- antigens
 - Pesticides
 - cold weather
 - fever
 - viruses
 - food
 - Smoking
 - germs


 

 antigens        