Fig. 7. Average unbinding force of individual LFA-1–ICAM-1 complexes as a function of force loading rate. Measurements were acquired using resting (open circle), PMA-stimulated (square) and Mg²⁺/EGTA-activated (closed circle) 3A9 cells at loading rates between 20 and 50,000 pN/second. This range of loading rates was achieved by varying the retraction rate of the cantilever from 0.1 to 15 µm/second and as a result of variations in the local compliance of the cell. This allowed for the effective spring constant of the cell-cantilever combination to have a range of values between 0.1 and 5 mN/meter. At fast cantilever retraction speeds (>1 µm/second), the hydrodynamic drag on the cantilever resulted in smaller forces recorded than were actually applied to rupture the LFA-1–ICAM-1 complex (Evans et al., 2001). The damping coefficient of the cantilever in the culture medium was 2 pN-second/µm. Each of the force spectra were acquired using five cells with an average of 100 measurements/cell. The error bar is the standard error.