Researchers identified a crucial role for air sac-based scavenger blood cells, alveolar macrophages (AMs), in driving hepatocellular cancer metastasis in the lungs. AMs were found to be recruited by interstitial macrophages (IMs) through interactions between the IM-expressed signaling molecule CCL2 and its AM-expressed receptor. AMs then secrete leukotriene B4, which promotes tumor cell growth. This is the first evidence for the distinct roles of AMs and IMs in liver cell lung metastasis.