The receptor is not a free-fit knob: θ(c) uses the CXCL8–CXCR1 affinity reported by Nasser et al. 2009 (Kd = 2.9 nM). Faint points trace every responder's (concentration, occupancy) over time; bold points are the final-frame survivors. With hill=2 and a θ=0.5 switch, the vertical Kd line and the horizontal threshold meet on the curve — cells to the right of Kd cross into the activated (amber) state, and those are the cells that chemotax and are recruited.