Anti-prothrombin (anti-PT) antibodies are found in antiphospholipid patients, but how they interact with prothrombin remains elusive. Prothrombin adopts closed and open forms. We recently discovered Type-I and Type-II antibodies and proposed that Type-I recognize the open form. In this study, we report the discovery, structural and functional characterization in human plasma of a Type-I antibody, POmAb. Using surface plasmon resonance and single-molecule spectroscopy, we show that …