The CD41 (HIP11) antibody recognizes a 140-kDa glycoprotein which is the α subunit of the CD41/CD61 (GPIIb/IIIa, αIIbβ3) complex called glycoprotein IIb (GPIIb). GPIIb is a calcium-dependent, no covalently associated heterodimer and contains a heavy chain (GPIIbα) and a light chain (GPIIbβ) linked by a single disulfide bond. The CD41 antigen is strictly expressed on platelets and platelet precursors (megakaryocytes). The CD41/CD61 complex is the receptor for fibrinogen, fibronectin and von Willebrand factor, and plays a central role in platelet activation and aggregation. The GPIIb/IIIa may be absent or strongly reduced in Glanzmann’s thrombasthenia (GT). The CD41 (HIP8) antibody inhibits platelet aggregation and ATP secretion induced by ADP, thrombin and collagen. The CD41 (HIP11) antibody reacts with α and β subunits of GPIIb/IIIa (CD41/CD61) complex.