So the air passageways of the lungs contain rings of muscle. When the passageways are irritated by, let's say, some type of allergen these muscles can constrict. Now, although this does make the airway more narrow and this decreases the likelihood that our pathogen will actually end up in our lungs and in our body, specifically in the alveoli of our lungs. It also actually has a bad effect because if we constrict, if we narrow in our passageways, that will make it very difficult for us to actually breathe. Now, the most common protective capability that you're probably familiar with is the process of coughing. And this is the process by which some type of irritant, when it gets into our lungs, into our passageway, it causes this reflex we call coughing.