Lung Immunology and Host Defense

Abstract

The respiratory tract is potentially exposed to a diverse array of gases, particulates, and microorganisms via inhalation or aspiration. Because the respiratory tract is directly exposed to substances in the surrounding environment, numerous defense mechanisms protect this interface between the external environment and internal tissues (Table 18.1). Inhalation of organic or inorganic dusts, exposure to toxic gases, aspiration of gastric acid, smoke inhalation, and inhalation or aspiration of infectious agents can induce lung disease. Whether foreign substances cause lung disease is determined by (1) patterns of deposition in the respiratory tract; (2) the total burden of inhaled or aspirated substances; (3) the adequacy of clearance mechanisms; (4) the adequacy of mucosal protective mechanisms; and (5) the presence of properly functioning immune responses to foreign substances. A given agent may cause no disease in some individuals, disease limited to the conducting system (tracheobronchial tree), or disease involving the distal lung parenchyma (gas-exchange areas). Because defenses that do not require antigen-specific inflammatory responses represent an important first line of defense that is backed up by specific immune reactions, this chapter will review the complex and integrated system of mechanical barriers and phagocytic defenses that exist in the respiratory tract in addition to specific immune responses that protect the lung from injury mediated by exogenous agents.