Mast cell activation in vivo is α2β1 integrin dependent. (A,B) Wild-type (WT) and α2 integrin–deficient (KO) mice were injected with zymosan intraperitoneally. At 1 hour after injection, peritoneal exudates were analyzed for the percentage of degranulated mast cells. Representative images of WT (scored as degranulated) and KO (scored as nondegranulated) PMCs are shown. Note the abundance of zymosan particles contained in nearby macrophages. Shown is the combination of 2 experiments, with each point representing an individual mouse.