Our lungs bring in vital oxygen and expel carbon dioxide. But they’re also an important immune site. They filter the air we breathe, repulsing invaders and repairing injury. But sometimes these powerful immune responses overreact, causing diseases such as asthma. Go to the Nature Immunology homepage: http://www.nature.com/ni/index.htmlNature has full responsibility for all editorial content, including Nature Video content. This content is editorially independent of sponsors.