Genetic disruption of Pak1 diminishes PCA in vivo. Evans blue extravasation after antigen challenge. (A,B) Pak1^−/− or WT mice (n = 5 genotype) were sensitized by intradermal injection of anti-DNP IgE (1:44 dilution, 1 μg/mL) into the right ear and with PBS into the left ear. After 20 hours, mice were challenged by intravenous injection of antigen (DNP-HSA) in PBS/Evans blue. IgE-primed (right) ears 30 minutes after antigen challenge are shown qualitatively (bottom). From each ear, Evans blue was extracted and the intensity of the dye was measured by absorption at 620 nm. *P < .05, WT versus Pak1^−/−, unpaired, 2-tailed, Student t test. (C) Mast cell–deficient W^sh/W^sh mice (n = 5) were reconstituted locally in the ears by intradermal injection of WT or Pak1^−/− mast cells. The mice were then sensitized by intradermal injection of anti-DNP IgE into the right ear and with PBS into the left ear as in panel A. IgE-primed (right) ears from mice reconstituted with WT or Pak1^−/− mast cells 30 minutes after antigen challenge are shown qualitatively (bottom). From each ear, Evans blue was extracted and the intensity of the dye was measured by absorption at 620 nm. *P < .05, WT versus Pak1^−/−, unpaired, 2-tailed, Student t test.