When macrophage digestion goes wrong

The exchange of gases between the air we breathe and our blood takes place via alveoli—tiny air sacs in our lungs. For this process to run smoothly, the epithelial cells of the alveoli produce a substance called "surfactant" that covers the alveoli like a film. This complex consists mainly of phospholipids and proteins and serves to reduce the surface tension of the alveoli. It also acts like a filter, reliably trapping bacteria and viruses that enter the lungs when we inhale.