Past the escalator: the alveolar frontier
The mucociliary escalator stops where the conducting airways end. The alveoli themselves have no cilia and no mucus — they cannot, because a sticky blanket there would block gas exchange. So the deepest, most delicate part of the lung needs a completely different kind of defense: cells, not conveyors. Anything tiny enough to ride the airstream all the way down (think fine smoke or fungal spores around 1–3 micrometers) arrives here, on the doorstep of the bloodstream.
Three defenders in the alveolus
The alveolar surface is guarded by a small team that works together.
- [[alveolar-macrophage|Alveolar macrophages]] are the resident big-eaters. They crawl over the alveolar lining, engulf inhaled particles and microbes, digest them, and either carry the debris back up to the escalator or migrate into the lymphatics. They are the lung's quiet, full-time janitors.
- The [[airway-epithelium|airway epithelium]] is not just a wall but an alarm system: its tight cell-to-cell junctions keep microbes out of the tissue, and its cells release signaling molecules and antimicrobial peptides that recruit help when they sense danger.
- [[pulm-surfactant|Surfactant]], made by type II pneumocytes, does more than reduce surface tension: some of its proteins coat microbes to make them easier to engulf and help calm inflammation. This immune side of surfactant is surfactant immunity.
The lung is not sterile
For a long time, healthy lungs were thought to be sterile. We now know they host a sparse but real community of microbes — the lung microbiome. Compared with the gut, it is thin and constantly turning over: microbes are continually inhaled and micro-aspirated from the mouth, and continually cleared by the defenses in this track. The lung microbiome is best understood as a balance of immigration and elimination, not a fixed garden.
Why does this matter? When defenses falter or the airways become diseased, this balance shifts. In chronic lung diseases the microbiome becomes less diverse and more dominated by a few aggressive species, which can drive ongoing inflammation. So the microbiome is both a marker of how well the defenses are working and, when disturbed, a player in disease itself. The next guide turns to exactly that — what happens, step by step, when these defenses fail.