Does the fluid-filled sac around the lungs function merely as a cushion from external damage?

It was long thought that the fluid-filled sac around our lungs functions merely as a cushion from external damage. Turns out, it also houses potent virus-eating cells that rush into the lungs during flu infections. These cells are macrophages, which are immune cells produced in the body, and go into the lungs when there’s an infection. A study found how during an influenza infection, the macrophages leave the exterior cavity and cross into the lungs where they decrease inflammation and reduce levels of disease. Using a laser-based technique, researchers tracked macrophages going into the lungs of mice, and found that the disease became worse with more lung inflammation when the macrophages were removed from the mouse. Once the protein signals have been identified, it may be possible to create drugs that boost either the number of macrophages, or their activity. The results show that macrophages are recruited to the lung during a severe influenza A virus infection and contribute to recovery from influenza. The researchers have been able to identify the source of lung macrophages independent of monocyte recruitment and local proliferation