Glycoprotein Ib (GPIb), also known as CD42, is a component of the GPIb-V-IX complex on platelets. The GPIb-V-IX complex binds von Willebrand factor, allowing platelet adhesion and platelet plug formation at sites of vascular injury.
It is deficient in the Bernard-Soulier syndrome. A gain-of-function mutation causes platelet-type von Willebrand's disease.
Autoantibodies against Ib/IX can be produced in immune thrombocytopenic purpura.
Components include GP1BA and GP1BB.
It complexes with Glycoprotein IX.