GPIb

GPIb is part of the GPIb-IX-V complex expressed on the platelet surface. When vascular endothelium is damaged, subendothelial collagen is exposed and immediately binds von Willebrand factor (vWF), which unfolds under shear stress. GPIb on platelets then binds this immobilized vWF, allowing platelets to adhere to the vessel wall despite high shear stress in flowing blood—this is the first step of primary hemostasis ('platelet adhesion'). This interaction is distinct from platelet aggregation, which is mediated by GPIIb/IIIa binding fibrinogen to link platelets together.

Clinical relevance: Bernard-Soulier syndrome is an autosomal recessive disorder caused by a deficiency or dysfunction of GPIb (or the GPIb-IX-V complex), resulting in impaired platelet adhesion. Patients present with mucocutaneous bleeding, a prolonged bleeding time, and thrombocytopenia with giant platelets on peripheral smear. Platelet count is often low, distinguishing it from Glanzmann thrombasthenia (GPIIb/IIIa deficiency), which has normal platelet count and morphology but absent aggregation.

Laboratory distinction: Ristocetin-induced platelet aggregation is used to test GPIb-vWF interaction. In Bernard-Soulier syndrome, ristocetin-induced aggregation is abnormal and does NOT correct with addition of normal plasma (distinguishing it from von Willebrand disease, where it corrects with normal plasma/vWF but not necessarily in all subtypes). This is a classic USMLE testable point regarding differentiating platelet adhesion defects.

Mnemonic: Bernard-Soulier = Big Suckers (giant platelets), Big problem with adhesion (GpIb deficiency), decreased platelet count.