Characterization of the FXI inhibitor. (A) Binding of IgG (4 mg/mL) from a healthy control, the herein reported patient, and a patient with severe inherited FXI deficiency with a known inhibitor to plates coated with purified FXI. (B) Western blots at different time points from reaction mixtures of purified FIX with FXIa and calcium ions, incubated in the presence of control IgG and patient's IgG (4 mg/mL). Note the cleavage of FIX in the presence of control IgG and lack of cleavage in the presence of the patient's IgG.