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The natural history of the immune response to exogenous factor VIII in severe haemophilia A

Summary. The development of inhibitory antibodies to factor VIII (fVIII) in severe haemophilia A patients is a serious therapeutic complication. Using a highly sensitive immunoprecipitation (IP) assay which measures all anti‐fVIII antibodies, we have tested severe haemophilic plasmas from two clinical studies. Inhibitor titres in the range of 0.4 to 1 Bethesda units/ml (BU/ml) could not be verified by IP as being due to an immune response to fVIII in 35% of plasmas tested. Low fVIII recoveries were likewise correlated with the presence of antibodies in 29% of plasmas tested. However, 16% of plasmas without inhibitor titres had immune responses as measured by IP. The rapidity of antibody appearance did not allow their effective detection by IP before development of inhibitor titres. These results suggest that the IP assay can provide a valuable confirmation of anti‐fVIII antibody production when the Bethesda assay is low or negative and where clinical observations suggest their presence, but they cannot be used reliably to detect early immune responses.