Pulmonary host defenses prevent subglottic airways from bacterial colonization. Several mechanisms are involved. Some of these are purely mechanical or anatomical (cough, mucociliary clearance, epiglottis larynx). Others are rather biochemical (bactericidal and anti-adhesive properties of oropharyngeal and tracheobronchial secretions) or immunological (opsonization and phagocytosis). Distal airways and alveolar defenses are mainly conducted by alveolar macrophages which are directly involved as effectors of cell mediated immunity. They also have a key role in regulation of local and systemic immune responses. In critically ill patients failure of this mechanisms of defense is common, therefore predisposing to development of nosocomial infections.