Lower respiratory tract infection
The principal function of the respiratory tract is gas exchange. It is therefore expose to the gaseous environment continuing particulate organic material such as bacteria, viruses and spores.
The entire respiratory tract is constantly exposed to air but the majority of particles are filtered out in the nasal hairs and by inertial impaction with mucus-covered surfaces in the posterior nasopharynx.
The epiglottis, its closure reflex and the cough reflex all reduce the risk of microorganisms reaching the lower respiratory tract.
Particles small enough to reach the trachea and bronchi stick to the respiratory mucus lining their walls and are propelled towards the oropharynx by the action of cilia (the mucociliary escalator).
Antimicrobial factors present to respiratory secretions further disable inhaled microorganisms.
They include lysozyme, lactoferrine and secretory IgA.
Particles in the size range 5-10 um may penetrate further into the lungs and even reach the alveolar air spaces.
Alveolar macrophages are available there to phagocytes potential pathogens. If these are overwhelmed, neutrophils can be recruited via the inflammatory response.
The defenses of the respiratory tract are a reflection of its vulnerability to microbial attack.
Acquisition of microbial pathogens is primarily by inhalation, but aspiration and mucosal and haematogenous spread also occur.
Individuals with healthy lungs rarely have any bacteria beyond the carina.
Respiratory pathogens have developed a range of strategies to overcome host defences. Influenza virus for example has specific surface antigens that adhere to mucosal epithelial cells.
The virus also undergoes periodic genetic reassortment , resulting in expression of novel adhesions to which the general population has no effective immunity.
Streptococcus pneumonia and Haemophilus influence both produce an enzymes (IgA protease) capable of disabling mucosal IgA.
Both of these species , other capsulated bacteria and mycobacteria are all resistant to phagocytosis. Penetration of local tissue is usually required before damage occurs. Although viruses using the common cold appear to be an exception.
Lower respiratory tract infection
