In medicine, glycoprotein IIb/IIIa (GPIIb/IIIa, also known as integrin αIIbβ3) is an integrin complex found on platelets. It is a receptor for fibrinogen and von Willebrand factor and aids platelet activation. The complex is formed via calcium-dependent association of gpIIb and gpIIIa, a required step in normal platelet aggregation and endothelial adherence. Platelet activation by ADP (blocked by clopidogrel) leads to the aforementioned conformational change in platelet gpIIb/IIIa receptors that induces binding to fibrinogen. The gpIIb/IIIa receptor is a target of several drugs including abciximab, eptifibatide, and tirofiban.