Anti-fXI monoclonal antibodies. Western blots of fXI and PK using (A) anti–human fXI IgG O1A6 or (B) anti–murine fXI IgG 14E11 as primary antibody. H indicates human fXI; M, murine fXI; A1, A2, A3, and A4, human fXI with PK domains A1, A2, A3, or A4, respectively; and PK, human PK. Positions of molecular mass standards are shown on the left. The uppercase D and M to the right of each panel indicate positions of fXI dimer and monomer (no interchain disulfide bond), respectively. Note that fXI/PKA4 is half the molecular mass of other fXI species because the fXI A4 domain mediates dimer formation. (C) Activation of 25 nM fXI with 5 nM fXIIa (○, ●) or 15 nM α-thrombin (□, ▵) in the presence (●, ▵) or absence (○, □) of 100 nM IgG 14E11.