Lung Defence Mechanisms and Their Potential Role in the Prevention of Ventilator Associated Pneumonia

Ventilator associated pneumonia remains a cause of significant morbidity and mortality in Intensive Care patients despite advances in knowledge and technology. The presence of an endotracheal tube bypassing the normal airway barriers, oropharyngeal bacterial colonisation, patient position and repetitive micro-aspiration tip the host-pathogen relationship in favour of the pathogen and promote lung infection in a time-dependent manner. Defending the lung against microbial invasion and infection is a multiplicity of innate and adaptive immune mechanisms, which can identify and eliminate potential pathogens. These defence mechanisms include the ability to recognise pathogens through cell-surface receptors, antimicrobial peptides and proteins, pro and anti-inflammatory mediators and factors related to the coagulant state of the lung fluid. The mechanisms are characterised by their complexity, their remarkable degree of redundancy and effectiveness.