Peritoneal mast cells express α2β1 integrin. (A,B) Peritoneal exudate cells from unmanipulated wild-type (WT, panel A) and α2 integrin–deficient (KO, panel B) mice were stained with PE–anti–c-kit and APC–anti–α2 integrin and assessed by flow cytometry. Mast cells were identified as c-kit^high-staining cells and represented 1%-3% of resident peritoneal cells in both WT and KO mice. (C,D) BMMC from WT (C) and KO (D) mice at 4 weeks of culture were stained with PE–anti–c-kit and APC–anti–α2 integrin and assessed by flow cytometry. Expression of the α2β1 integrin was not detected. (E) Representative images of PMCs from WT and KO mice stained with Wright-Giemsa stain. Cells were of equal size and contained large numbers of basophilic staining granules. (F) PMCs from WT and KO mice were stimulated with Compound 48/80 at a range of concentrations for 1 hour. Percent serotonin release was calculated (mean ± SEM). (G) PMCs from WT and KO mice were assayed for adhesion to BSA, type I collagen, or fibronectin. Results are presented as means ± SEM.