Lung defenses against infection: a clinical correlation

The current state of knowledge of lung defenses has been reviewed. First, mechanical factors such as aerodynamic filtration and mucociliary transport were considered. Then, in general terms, the contributions of alveolar macrophages, neutrophils, lymphocytes, and immunoglobulins, and the roles of complement, antiproteases, lysozyme, and fibronectin were examined. Interactions between these components may regulate their effect. Finally, the responses to five specific microorganisms were reviewed to illustrate different aspects of the lung's defenses. Streptococcus pneumoniae was selected as a representative extracellular bacterial pathogen, Mycobacterium tuberculosis as an intracellular bacterial pathogen, Mycoplasma pneumoniae because it elicits significant humoral and cell-mediated immunity, respiratory syncytial virus as an example of a local viral pathogen, and measles as a viral pathogen that causes generalized disease. It was shown that these responses may not always be beneficial for the host. For each of the five infections, recommendations for improving the outcome were made.