LFA-1 and Mac-1, two β2 integrin members constitutively expressed on neutrophils, mediate leukocyte recruitment cascade by binding to the same ligand of ICAM-1The slow rolling and firm adhesion of leukocytes rely on LFA-1 while the cell crawling is dependent on Mac-1We hypothesized that their distinct roles are likely attributed to the differences in the binding kinetics or in the diverse responses of outside-in and inside-out signalingIn this study, we compared the ICAM-1 binding features between soluble or membrane-expressed LFA-1 and Mac-1 with different affinity conformation using optical trap techniqueOur data indicated that the affinity up-regulation from wide type (WT) to high affinity (HA) is off-rate dependent for LFA-1 but on-rate dependent for Mac-1The structural bases of this new finding were found to be consistent with our previous simulationsThese results furthered our understanding in their function differences under shear flow.