Analysis of binding of patients' IgG to factor XIa by Western blotting. Factor XIa was electrophoresed on 8% SDS-PAGE (0.2 μg per lane) under reducing conditions, followed by transfer to a PVDF membrane. Each lane was then incubated with the IgG of the patients (lanes 1-4), and one lane was incubated with goat antihuman factor XI IgG (control). The lanes were then processed as described in “Patients, materials, and methods.” The 50-kDa heavy chain (HC) and the 30-kDa light chain (LC) are depicted. Normal control IgG and IgG purified from a type II homozygote with no inhibitor failed to recognize factor XI (data not shown).