GPIb receptor

GPIb is part of the GPIb-IX-V complex on the platelet membrane. When vascular endothelium is damaged, collagen is exposed and vWF binds to this collagen. GPIb on platelets then binds vWF, allowing platelets to adhere to the vessel wall despite high shear stress (this is the first step of primary hemostasis, distinct from platelet aggregation which uses GPIIb/IIIa binding fibrinogen). After adhesion, platelets become activated, release granule contents (ADP, thromboxane A2), and undergo a conformational change exposing GPIIb/IIIa receptors for aggregation.

Clinical correlations: - Bernard-Soulier syndrome: autosomal recessive deficiency/dysfunction of GPIb receptor, leading to defective platelet adhesion. Presents with mucocutaneous bleeding, prolonged bleeding time, thrombocytopenia with giant platelets, and normal platelet count aggregation studies showing absent response to ristocetin (since ristocetin-induced agglutination requires GPIb-vWF interaction) but normal response to ADP/collagen. - Contrast with Glanzmann thrombasthenia: deficiency of GPIIb/IIIa, impairing platelet aggregation (not adhesion); ristocetin-induced aggregation is normal but ADP/collagen-induced aggregation is absent. - von Willebrand disease: deficient or defective vWF impairs GPIb binding, causing similarly absent ristocetin cofactor activity, but the platelet receptor itself is normal (differentiate via mixing studies with normal plasma vWF, which corrects the defect in vWD but not in Bernard-Soulier).

Understanding the GPIb-vWF-collagen axis is essential for distinguishing qualitative platelet disorders on USMLE, especially via ristocetin cofactor assay reasoning.