CD42b, also known as platelet glycoprotein Ib alpha chain （GPIbα）, is a type I transmembrane glycoprotein with a molecular weight of approximately 145 kDa. It is encoded by the GP1BA gene and primarily expressed on the surface of platelets and megakaryocytes. CD42b forms a complex with other proteins, including CD42c （GPIbβ）, CD42a （GPIX）, and CD42d （GPV）, which is essential for platelet adhesion and aggregation. This complex plays a crucial role in hemostasis by binding to von Willebrand factor （vWF）, facilitating platelet attachment to the subendothelium at sites of vascular injury. Mutations or deficiencies in CD42b are associated with Bernard-Soulier syndrome, a rare bleeding disorder characterized by large platelets and impaired platelet function.