inflammation of the pleura, the membranes that line the thoracic cavity and fold in to cover the lungs (lung). Pleurisy may be characterized as dry or wet. In dry pleurisy, little or no abnormal fluid accumulates in the pleural cavity, and the inflamed surfaces of the pleura produce an abnormal sound called a pleural friction rub when they rub against one another during respiration. This rubbing may be felt by the affected person or heard through a stethoscope applied to the surface of the chest. In wet pleurisy, fluids produced by the inflamed tissues accumulate within the pleural cavity, sometimes in quantities sufficient to compress the underlying lung and cause shortness of breath. Because the pleura is well supplied with nerves, pleurisy can be very painful. Pleurisy is commonly caused by infection in the underlying lung and, rarely, by diffuse inflammatory conditions such as lupus erythematosus. Treatment of pleurisy includes pain relief, fluid evacuation, and treatment of the underlying disease.