NBT-reduction for detection of specific, opsonizing antibodies against haemophilus parainfluenzae

Specific, opsonizing antibodies against Haemophilus parainfluenzae were demonstrated in serum by measuring their enhancing effect on the nitroblue tetrazolium (NBT)-reduction of normal human granulocytes, phagocytizing the homologous bacteria. Patients with septicemia and/or endocarditis and carriers of H. parainfluenzae were studied. In late phase sera of the first group such antibodies were detected in 6 of 9 cases. Opsonizing antibodies against H. parainfluenzae were not detected in any of the carrier sera. A dose-response effect was observed when a positive late phase serum was tested in serial 2-fold dilutions. The enhancement of the NBT-reduction was eliminated by absorption of serum with homologous antigen. The detection of opsonizing antibodies, essential for the immune defense against H. parainfluenzae might be helpful for evaluation of the pathogenicity of this organism, which so far is considered as a rare causative agent.