Tf and A24 partially compete for binding to TfR. Experimental conditions are described precisely in “Materials and methods.” (A) Tf was injected across the TfR surface, after saturation with A24 (bold line) or a control buffer injection (thin line). (B) A24 was injected across the TfR surface, after saturation with Tf (bold line) or a control buffer injection (thin line). (C-D) The apparent stability of the complex between A24 and TfR is strongly dependent on the density of TfR. A series of concentrations of Fe-Tf and A24 was injected across a low-density (C, 120 RU) or a high-density (D, 1650 RU) immobilized TfR surface; only the data obtained for 134 nM of Fe-Tf (thin line) and 62.5 nM of A24 (bold line) are shown. A24/TfR complexes dissociate faster than Tf/TfR when TfR is present at low density (C), whereas at high density, the opposite situation is observed (D and Table 1).