The HIP8 antibody reacts with the ~125 kDa GPIIbα subunit of human CD41, a heterodimeric integrin expressed primarily on platelets, megakaryocytes, monocytes, and acute megakaryoblastic leukemia cells. The GPIIbα subunit is linked to a smaller ~25 kDa subunit (GPIIbβ) by a single disulfide bond. Three isoforms of CD41 that are generated by alternative splicing have been identified, one of which is expressed only by cancerous cells. CD41 associates non-covalently with CD61 (integrin b3) in a calcium-dependent manner to form the CD41/CD61 (or GPIIb/IIIa) complex, which has been identified as a receptor for several ligands, including fibrinogen, fibronectin, vitronectin, and thrombin. Binding to the CD41/CD61 complex involves an RGD tripeptidyl sequence in the ligand, and both ligand occupancy and ligand-mediated receptor clustering are functionally important in the integrin-mediated response. The HIP8 antibody reportedly blocks platelet aggregation and inhibits the activation of platelets by several compounds.