[Immunity and pathophysiology of respiratory tract infections]

The respiratory tract is permanently exposed to infections that may remain localized (bronchitis, pneumonias) or become potentially invasive (bacteremia and meningitis). It can be considered as an immunologic organ the upper part of which, the tracheobronchial tree, has the same secretory epithelium as the naso-oropharynx and shares bronchial associated lymphoid tissue (BALT). In this tissue, secretory IgA are more abundant than IgG. It is colonized by a commensal bacterial flora, including some potentially pathogenic species (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis). The pulmonary compartment includes the bronchioles and the alveoli, the wall of which is made of pneumocytes, resident macrophages, plasmocytoid dendritic cells and T cells. This wall is protected by a film that contains microbicidal agents, such as surfactant and phospholipase A2. Immune defenses of the respiratory tract involve mechanical factors, mucociliary escalator, receptor and effector molecules of the innate immune system and, by the proximity of lymph and blood vessels, humoral and cellular effectors of adaptative immunity. However, this sophisticated respiratory tract immune system can be bypassed in the non immunized host by infections due to primary pathogens (tuberculosis, plague, whooping cough, influenza) and may be impaired by endogenous factors (genetic defects, iatrogenic disorders) or exogenous factors (chemical pollutants, respiratory viruses) making the host susceptible to occasional pathogens, including commensal organisms.