Leukocyte infiltration into cantharidin-induced skin blisters. Duplicate skin blisters were established by topical application of cantharidin (0.1%) to the forearm of 9 healthy individuals. Blister fluid was collected at 16 hours and analyzed: (A) for fluid volume and cellularity (each point represents the mean from 2 skin blisters; horizontal line represents the population median) and (B) by flow cytometry to distinguish leukocyte subpopulations. The representative dot plot in panel B shows gating into distinct granulocyte and mononuclear-cell populations by characteristic forward- and side-scatter properties. The inset shows CD16 and VLA-4 staining in the granulocyte gate, which allows neutrophils (CD16⁺VLA-4^–) and eosinophils (CD16^–VLA-4⁺) to be resolved and, furthermore, allows the proportion of apoptotic neutrophils (CD16^dimVLA-4^–) to be determined. CD14 staining in the FL-3 channel was used to resolve cells in the mononuclear-cell (MNC) gate into monocyte (CD14⁺) and lymphocyte (CD14^–) populations.