inhaled environment and the internal tissues. It forms a complex physicochemi­cal and mechanical barrier supplemented by mucociliary clearance to provide the rst line of defense against inhaled pathogens. The airway mucus covering the respiratory tract’s luminal surface entraps inhaled pathogens, including infectious agents, and eliminates them by the coordinated beating of the cilia. Epithelial cells maintain epithelial integrity by attaching to their neighbors by tight junctions, adherens junctions, gap junctions, and desmosomes [7]. These structures form an impermeable and effective mechanical barrier to most patho­gens and permit the maintenance of an ionic gradient for the directional exchange of many nutrients, water, and gases [7, 8]. The upper airway epithelial cells ef­ciently prevent microparticles >10μm and a large percentage of particles >5μm from reaching the lower airways via direct prevention of their passage through the nasopharyngeal mucosa [8, 9]. The particles entrapped are either exhaled with cough or ingested with mucus into the gastrointestinal tract. If pathogens crack structural defenses, the innate immune system mediators of the upper respiratory tract, including salivary lysozyme, peroxidase, and lactoferrin, func­tion with antimicrobial activity. Lower respiratory system airways are also lined with epithelial cells and beating cilia. Approximately 90% of particles >2–3μm trapped within the mucus are transported through the mucociliary escalator from the bronchioles to the trachea and exhaled with the help of a cough [7].