Observations in allergen challenged animals demonstrating anti-Fas-induced mucus-exudate plugs (A,C,D,E) compared to control isotype antibody (IgG) treatment (A,B). The occurrence of mucus-exudate plugs is expressed as percentage affected airways in each tissue section in control mice (white bars) and in mice treated with anti-Fas mAb (black bars) at 8 and 24 hours (A). Bars indicate the standard error of the mean for each group of animals (n = 8, ** = p < 0.01). Histochemical staining with periodic acid-Schiff reagent (PAS) illustrated mucus-containing cells (B,C) and so did the transmission electron micrograph (D). Tethered secretions and lumen plugs occurred foremost in anti-Fas treated airways (C,D;E). Fibrinogen immuno-reactivity was distributed in the mucus plugs exclusively in anti-Fas treated airways (E).