[A circulating inhibitor specific to factor V. Clinical, biological and therapeutic study (author's transl)].

A 65-year-old man developed severe haemorrhagic diathesis associated with complete and isolated factor V deficiency due to the presence of a circulating antibody inhibiting specifically Factor V. No evidence of autoimmune disease was found, but the patient had received isoniazid, streptomycin and PAS for pulmonary tuberculosis 12 years before the inhibitor was discovered. This low titer (4 micro/ml) was an IgG of the gamma G1, gamma G2, or gamma G4 subclass. It was stable at 56 degrees C but unstable at low pH, had strong affinity for human plasma Factor V and did not cross-react with either porcine or bovine Factor V. Its inhibitory activity was blocked by both normal human plasma or serum, but not be the plasma of a patient with severe, inherited Factor V deficiency. A 4-week immunosuppressive treatment with prednisone and azathioprine resulted in disappearance of the antibody and return of Factor V to a normal level. However, this level rapidly decreased after immunosuppression was discontinued, and could only be maintained at 40 to 60% of normal values by longterm treatment with prednisone and cyclophosphamide.