This study elucidates the structural basis of allostery in integrins and their binding to fibrinogen-mimetic therapeutics. Using crystal structures, the research defines the atomic mechanisms underlying conformational changes and ligand affinity in the integrin ectodomain, particularly focusing on the platelet integrin αIIbβ3. Allostery in the β3 I domain alters three metal binding sites, associated loops, and α1- and α7-helices, leading to a 62° reorientation between the β3 I and hybrid domains. This allostery transmits through the rigidly connected plexin/semaphorin/integrin (PSI) domain, causing a 70 Å separation between the α and β legs, which favors leg extension and positions the high-affinity head above the cell surface. The study reveals an open, high-affinity conformation of the αIIbβ3 headpiece, its binding to therapeutic antagonists, and the allosteric movements linking the ligand binding site of βI domains to α7-helix displacement and outward swing of the hybrid domain. The β3 hybrid and PSI domains act as a rigid lever, transmitting and amplifying this motion, resulting in a 70 Å separation between the α and β legs at their knees. The research also identifies the structural basis for ligand binding to αIIbβ3, showing that small molecule antagonists bind to a pocket formed by loops in the αIIbβ propeller and β3 I domain. The binding site for fibrinogen is more extensive, involving residues critical for ligand recognition. The study highlights the importance of the β3 specificity determining loop (SDL) and other loops in the αIIbβ propeller domain for fibrinogen binding. The study further demonstrates the structural differences between the αIIb and αVβ propellers, particularly in cap inserts 1, 2, and 3. The β3 SDL closely associates with the cap subdomain, and structural variations between αIIb and αV are responsible for different β3 SDL conformations. The research also reveals the structural basis for the allosteric mechanism of integrin activation, showing that structural rearrangements between closed and open conformations are general for all integrins. The study provides insights into the structural basis for drug binding and selectivity, highlighting the role of basic and acidic moieties in the fibrinogen sequence in drug binding. Overall, the study provides a detailed structural understanding of integrin conformational changes and their regulation, which has implications for the development of therapeutics targeting integrins. The findings contribute to the understanding of integrin function and the mechanisms underlying their activation and regulation.This study elucidates the structural basis of allostery in integrins and their binding to fibrinogen-mimetic therapeutics. Using crystal structures, the research defines the atomic mechanisms underlying conformational changes and ligand affinity in the integrin ectodomain, particularly focusing on the platelet integrin αIIbβ3. Allostery in the β3 I domain alters three metal binding sites, associated loops, and α1- and α7-helices, leading to a 62° reorientation between the β3 I and hybrid domains. This allostery transmits through the rigidly connected plexin/semaphorin/integrin (PSI) domain, causing a 70 Å separation between the α and β legs, which favors leg extension and positions the high-affinity head above the cell surface. The study reveals an open, high-affinity conformation of the αIIbβ3 headpiece, its binding to therapeutic antagonists, and the allosteric movements linking the ligand binding site of βI domains to α7-helix displacement and outward swing of the hybrid domain. The β3 hybrid and PSI domains act as a rigid lever, transmitting and amplifying this motion, resulting in a 70 Å separation between the α and β legs at their knees. The research also identifies the structural basis for ligand binding to αIIbβ3, showing that small molecule antagonists bind to a pocket formed by loops in the αIIbβ propeller and β3 I domain. The binding site for fibrinogen is more extensive, involving residues critical for ligand recognition. The study highlights the importance of the β3 specificity determining loop (SDL) and other loops in the αIIbβ propeller domain for fibrinogen binding. The study further demonstrates the structural differences between the αIIb and αVβ propellers, particularly in cap inserts 1, 2, and 3. The β3 SDL closely associates with the cap subdomain, and structural variations between αIIb and αV are responsible for different β3 SDL conformations. The research also reveals the structural basis for the allosteric mechanism of integrin activation, showing that structural rearrangements between closed and open conformations are general for all integrins. The study provides insights into the structural basis for drug binding and selectivity, highlighting the role of basic and acidic moieties in the fibrinogen sequence in drug binding. Overall, the study provides a detailed structural understanding of integrin conformational changes and their regulation, which has implications for the development of therapeutics targeting integrins. The findings contribute to the understanding of integrin function and the mechanisms underlying their activation and regulation.