The Xia.B2 antibody reacts with mouse GPIb alpha (CD42b), a platelet/megakaryocyte-specific polypeptide that is part of the GPIb-V-IX complex, a receptor for different ligands, including vWf, thrombin, and P-selectin. Xia.B2 blocks the vWf on GPIb alpha, which is located on the N-terminal 45 kDa domain of the receptor.