The differential binding of 2 monoclonal anti-FXIII-B antibodies to complexed and free FXIII-B. (A) FXIII-B was immunoprecipitated from human plasma by 2 monoclonal anti-FXIII-B antibodies and the presence of FXIII-B and FXIII-A, and the immunoprecipitates were detected by western blotting using polyclonal anti-FXIII-B or anti-FXIII-A antibodies on separate blots. FXIII-A was present in the immunoprecipitate obtained by anti-FXIII-B(1) but absent when anti-FXIII-B(2) was used for immunoprecipitation. (B) the concentration-dependent inhibition of FXIII-A₂B₂ complex formation by anti-FXIII-B(2), but not by anti-FXIII-B(1), antibody was demonstrated in an ELISA-type assay as described in “Effect of anti-FXIII-B antibodies on the interaction of FXIII subunits” in “Methods.” The label on the y-axis (optical density [OD] 450 nm) is a measure of FXIII-A₂B₂ complex formation.