Monoclonal antibody 3B1 recognizes human coagulation factor V (FV; proaccelerin). The coagulation pathway is formed by sequential enzymatic activation of serine proteases leading to the generation of thrombin. Regulation of this pathway is essential in order to prevent bleeding and unnecessary clotting. FV is an essential clotting factor with little or no intrinsic procoagulant activity until the conversion to FVa through proteolysis by thrombin or activated factor X. About 80% of blood FV is derived from the liver, the remaining 20% is stored in platelet α-granules. This is released after platelet activation. FV is a large plasma glycoprotein of ca 330KD. After cleavage into FVa a heavy and light chain is generated which is held together by calcium ions. FV contains many posttranslational modifications, which are important for the procoagulant and anticoagulant function of FV and FVa. Defects in FV may lead to hemorrhagic or thrombotic phenotypes. Most well-known is FV-Leiden mutation, in which AA R506 has been mutated to Gln. Due to this mutation FVa activity is downregulated by activated protein C.