kd=2.9 nM · conc_scale=0.02 · hill=2.0  ·  Each responder carries a per-cell Hill-occupancy receptor gated by a cited CXCL8–CXCR1 Kd; only once occupancy crosses threshold does it switch from naive to chemotaxing. Forcing the activated-type response off (blocked) abolishes recruitment even though the cue is still present and receptors still bind it — isolating the response, not the cue, as the causal step. Ribbons are 95% CIs across 5 seeds.