P2X₇^−/− mice are resistant to allergic but not to irritant contact dermatitis. (A) C57BL/6 WT or P2X₇^−/− mice were sensitized by application of 3% TNCB in acetone or acetone alone (vehicle) on the shaved abdominal skin. Both ears were challenged on day 5 after sensitization with 1% TNCB. Ear thickness was measured at the time of challenge and 24 h later. (B) The experiment was performed as described in A, except with 3% oxazolone in ethanol or ethanol (vehicle). Ear challenge was performed on day 6 with 1% oxazolone in ethanol. (C) WT and P2X₇^−/− mice were treated with 1% croton oil in 4:1 acetone/olive oil or acetone/olive oil alone (vehicle) on the ear skin. The ear thickness was measured before and 24 h after treatment. (A–C) Results represent the increase in ear thickness in groups of five mice ± SD. One of two (C) or three (A and B) independent experiments is shown (***, P < 0.001). (D) Ear skin from TNCB-sensitized P2X₇^−/− and WT mice was stained with H&E 24 h after challenge with vehicle or TNCB.