Pulmonary host defense: coordinated interaction of mechanical. cellular and humoral immune systems of the lung

The respiratory system has numerous ways to protect lung parenchyma from implantation of bacteria and subsequent infection. Such natural defense utilizes the clearing mechanisms in the nose, larynx and upper airways and the cellular and humoral immune factors in the lower respiratory tract so effectively that the host is largely unaware of its tireless surveillance. Normal lungs are kept sterile. In contrast, pneumonitis, which represents the fully developed acute inflammatory reaction, signals the ultimate response to virulent bacteria, but can be considered as general failure of host defense also depending on the viewpoint taken. In between these extremes are gradations of response, which are unnoticed, perhaps. Thus, the inflammatory response in the lung to bacteria requires initiation, modulation and eventually suppression. This report attempts to dissect individual components of this response and to examine cell products which may have regulatory functions.