Fact 1: Abreathing mask is used for keeping harmful particles from entering the respiratory system.

Fact 2: Another respiratory defense is the immune system.

Given the two facts above, answer the question "the what is a defense against harmful particles from entering the body" with the following options: 
- lungs
 - It remains dormant
 - A virus
 - immune system
 - exoskeleton
 - pancreas
 - chickenpox
 - liver


 

    immune system     