PULMONARY IMMUNE CELLS IN HEALTH AND DISEASE - POLYMORPHONUCLEAR NEUTROPHILS

The pulmonary vasculature represents the largest reservoir of polymorp honuclear neutrophils (PMNs) in the human body. This is in striking co ntrast with the paucity of PMNs present in the normal airways and alve oli. However, the respiratory tract constitutes an easy access for mic roorganisms and particles present in inhaled air and, therefore, effic acious defence mechanisms are required. When the mucociliary clearance and the alveolar macrophages are over-whelmed, the rapid recruitment of PMNs from the lung vasculature appears to be a crucial response of the host against the pathogens. The regulation of adherence of PMNs to endothelial cells (EC), followed by the transendothelial migration ar e now better understood, and are under the control of a series of adhe sion molecules modulated by bacterial and inflammatory mediators. In a ddition to their defensive role, PMNs have also been implicated in acu te and chronic injurious diseases of the lung. Clearly, PMNs contain e nough cytotoxic and proteolytic material to induce lesional changes. H owever, the release of this material is likely to be dependent on envi ronmental factors, including mediators derived from other inflammatory and immune cells. The presence or absence of these factors could expl ain the fact that high numbers of PMNs can be observed in the airways and alveoli without major lesions whilst in other conditions, a margin al increase of PMNs in the respiratory tract can be associated with ma jor damage and irreversible architectural changes in the lung.