I. Defense Mechanisms of the Pleura The pleura is a monolayer of mesothelial cells intricately connected with the underlying lung and tissues through a network of balancing cellular and humoral factors that allow for host defense of the pleural space. The pleural membrane not only serves as a barrier function, but also has multiple other defense mechanisms that are focused on maintaining the homeostatic balance of the pleural space. Because the pleura encircles a closed potential space, it does not interface with the external environment as does the lung (1). Changes in the delicate homeostatic balance of the pleural space can be initiated by the presence of foreign cells, proteins, or microbes. Even the presence of air in the pleural space changes this balance.