What defense does the respiratory system have to keep it healthy?
Mucus, Cilia cells
Mucus is by far the best defence mechanism the respiratory system has to protect itself against anything foreign that gets into your tracts. It is produced by cells in the superior respiratory pipes, and prevents any solid or liquid particle from entering the inferior respiratory pipes, where gas exchanges happen. The mucus grabs onto any infiltrated particle (dust, bacteria, liquids, or any foreign object). Cilia cells allow this mucus to be evacuated and not remain stationary within the respiratory pipes.
This is why when you are sick, your mucus production is increased to evacuate any foreign bacteria/virus that might have infiltrated in your wind pipes.