Fibroblast
Fibroblast

The "fibroblast" or "fibroblastic" focus (ff) is seen at the interface between densely fibrotic lung tissue and adjacent normal lung in UIP. The fibroblasts in the focus are arranged parallel to the surface, and reactive type 2 cells (arrows) form a surface layer over them. The fibroblast focus is felt to be the site of damage or injury in UIP.