Introduction

Despite the deceiving appearance of being clean, the air we breathe contains a variety of smokes, pollens, hazardous chemicals, debris and microorganisms. Airway contact with microbes is continuous. Although minor infections are frequent, it is surprising that serious infections such as pneumonia are relatively rare in the immunocompetent host. With an elaborate array of host defence mechanisms, the respiratory system is able to remain largely free from infection. A specific inciting event, either acquired or inherited, is needed to overcome this competent defence. Disruption of anatomic barriers, decreased clearance of secretions, depressed cough reflex and derangements or alterations in immune status are but a few of the mechanisms whereby this intricate system can be compromised (Table 3.1). The respiratory defence system acts differently from a usual surveillance mechanism in that it becomes active on a periodic basis when the need arises. Although the respiratory defence system is co-ordinated from the nose and mouth to level of the alveolar unit, it will be considered separately as the airways and the alveoli. Each has unique adaptations to resist and combat infection which are integrated to maintain a disease-free state.

Airway structure

The airways of the respiratory system are covered by a mucosa which varies in cell type throughout the system. The airway surface is composed of three distinct layers: an epithelium, the basement membrane and the submucosa.