Human prothrombin activation.

Antiserums to human prothrontbin contain antibodies directed against at least two different antigenic sites. During blood coagulation, prothrombin is cleaved into two major antigenically distinct fragments-thrombin and a "pro" fragment. The latter is present in normal seruim, whereas thrombin loses its immunologic reactivity, presumably because of its combination with the natural thrombin inhibitors in blood.