Measurement of anti-factor IX IgG subclasses in haemophilia B patients who developed inhibitors with episodes of allergic reactions to factor IX concentrates.

We have established a simple enzyme-linked immunosorbent assay (ELISA) for the detection of anti-factor IX IgG subclasses in haemophilia B patients with inhibitors. The assay was performed using immobilized purified factor IX. Specific IgG subclasses were detected by peroxidase-conjugated anti-human IgG1,2,3 and 4. Ten plasma samples from 6 haemophilia B patients with inhibitors ranging from 1.0 to 253 Bethesda Units/ml were analyzed. All samples were positive for IgG4. Six out of 10 samples were positive only for IgG4. Three samples were positive for IgG2. Five of the 6 patients had previously had allergic reactions to factor IX concentrates. Three patients had allergic episodes within the past month. Three samples from these latter patients taken on the day when the allergy had occurred showed positive also for IgG1. In later samples, however, taken at 4 days and 4 weeks respectively from two of these same patients. IgG1 was not detected. In two of the five patients in whom allergic reactions had occurred more than one month previously IgG1 was not detected. The results suggested that allergic reactions in patients with haemophilia B treated with factor IX concentrates were associated with the development of the specific IgG1 subclass of antibody to factor IX.