The p75 TNF-α receptor is essential for TNF-α–induced firm adhesion. (A) Representative leukocyte firm adhesion profile following mouse TNF-α treatment of WT (○), p75^−/− (▪), p55^−/− (▴), and D^−/− (□) mice. One hour after surgery, TNF-α (2 ng/mL in 25 μL) was applied directly to a 25-mm² area of the cremaster muscle flap. Number of adhering leukocytes was counted in a time window of 2 minutes in a 100-μm viewing segment. Control counts were obtained prior to TNF-α application. (B) Average fold change with respect to control in early- and late-phase firm adhesion of leukocytes upon TNF-α treatment of WT (filled bar), p75^−/− (spotted bar), p55^−/− (hatched bar), and D^−/− (shaded bar) cremasters. Group size equals 4, error bars represent SD, the asterisk indicates statistical significance (P < .05).