Purified human prothrombin was from Synapse Research Institute (Maastricht, The Netherlands) and rivaroxaban (Xarelto) was from Bayer Schering Pharma AG. Anti‐FcγRIIA monoclonal antibody (mAb) IV.3 was purified in the laboratory from a hybridoma.12 (link) The antiprothrombin mAb 28F4 is a mouse IgG antibody, as described previously.13 (link), 14 (link) Antiprothrombin fragment 1+2 mAbs 3B1, 6A3, 11H2, and 8H11 were developed and produced according to standard procedures.15 (link) LAC was measured by activated partial thromboplastin time (Diagnostica Stago), dilute Russell Viper Venom Time (Diagnostica Stago), and Ecarin Clotting Time (ECT; Diagnostic Reagents). The Pierce Classic IP Kit from Thermo Fisher Scientific was used for immunoprecipitation. High‐binding enzyme‐linked immunosorbent assay (ELISA) plates from Costar were used for the ELISAs. Sheep anti‐human prothrombin IgG horseradish peroxidase–labeled antibody was from Affinity Biologicals, Inc., prothrombin fragment 1+2 was from Haematologic Technologies, Inc. All other reagents are previously described or are from Sigma‐Aldrich.16 (link)