Question

Larry is taking a walk in the park. The pollen in the air makes him sneeze constantly. What line of defense is his immune system exhibiting?

Answer 1

I think it is first line of defense. 

Answer 2

The line of defense that is his immune system exhibiting is the first line of defense. The first line of defense are a combination of physical and chemical barriers that prevent all types of foreign agents from penetrating the outer layer of the body. In this case, it is the nasal cavity.