Spontaneous development of VIII inhibitors in non-haemophiliacs is reported in 1 per million population. Antibody is usually IgG, occasionally IgM or IgA and will neutralise the functional VIII protein. In 15-25% of haemophilic patients antibodies develop as a result of treatment with factor VIII concentrates usually within the first 10-20 treatment exposures. A familial tendency is noted, inhibitors occurring more often in patients with deletions or mutations within factor VIII gene. The antibody acts against part of the amino-terminal component of the A2 domain or the carboxy-terminal part of the C2 domain of the VIII molecule. It may be quantitated by the Bethesda titre (BU; see below). Factor IX very rarely (<2%) stimulates antibody formation.

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