Recent studies suggest that factor (F) XII plays a key role in thrombus stabilization and growth but is dispensable for hemostasis. We have previously shown that histidine-rich glycoprotein (HRG), a protein present in platelets and plasma, binds FXIIa and inhibits FXII autoactivation and FXIIa-mediated activation of FXI, thereby downregulating thrombosis. HRG binds various ligands, including FXIIa, fibrin(ogen), nucleic acids and polyphosphate (polyP). Studies have shown that polyP, released from activated platelets, and artificial surfaces like catheters, can promote FXII activation. This suggests that HRG can downregulate the activation of the contact system. This thesis aims to determine the potential mechanisms by which HRG modulates platelet function and thrombosis induced by polyP or catheters. We show that HRG binds polyP with high affinity and inhibits the procoagulant, prothrombotic and cardiotoxic effects of polyP via at least two mechanisms. First, HRG binds polyP and neutralizes its procoagulant activities and cytotoxic effects. Second, HRG binds FXIIa and attenuates its capacity to promote autoactivation and activate FXI. Also, we identify that HRG serves as a molecular brake for the contact system by attenuating the procoagulant activity of FXIIa regardless of whether FXII activation is triggered systemically with polyP or occurs locally on the surface of catheters. Our studies have identified HRG as a novel ligand for platelet receptor GPIbα on resting platelets, and upon activation, it competes with fibrinogen for binding to GPIIb/IIIa integrin, thereby inhibiting platelet aggregation. These findings suggest that HRG may modulate coagulation as well as platelet function. Therefore, supplementation with HRG or HRG analogs may serve as a potential therapeutic option to attenuate polyP or catheter-induced thrombosis without perturbing hemostasis. / Dissertation / Doctor of Philosophy (PhD)