Kd=2.9 nM · conc_scale=0.02 · hill=2.0  ·  Each responder carries a per-cell Hill-occupancy receptor: it reads the local chemokine C, computes θ = c^h/(Kd^h+c^h), and switches from naive (grey) to activated (amber, chemotaxing) once θ ≥ 0.5. The source is mobile — its chemokine blob wanders and activated responders chase it, staying inside the recruitment radius, while naive or squeezed-out cells fall behind. Field colour is shown on a saturating 0–800 a.u. scale to reveal the gradient (peak ≈ 2.6k a.u.).