From the tonsils through the entire respiratory track, numerous lymphatic outposts guard the lungs from pathogens and allergens. The cells that line the respiratory tract secrete numerous molecular messages to monitor and control your body's response to the things you inhale. Goblet cells and bronchial glands secrete mucus. Lymphocytes are poised to mount an immune response to novel antigens. Plasma cells produce antibodies to coat foreign substances so they will be recognized and ingested by macrophages.
Mast cells produce histamine and other signal molecules.
Macrophages and lymphocytes produce factors that increase the intensity of the immune response. This marvelous system of cells is designed to protect the vital respiratory zone. The striking image above shows the conducting zones as well as the deep lung. As you can see, once you get past the filtration network of the first 16 branches you reach the alveoli and the smallest branches of the bronchial tree. The surface area of this highly absorbant layer is about the size of a tennis court.