Characterization of the bactericidal antibody response against Haemophilus influenzae.

Rabbits were used in a study of the bactericidal (BC) antibody response against Haemophilus influenzae of types a and b and their respective non-capsulated variant strains. Previously described methods, complement fixation and indirect hemagglutination, were used for comparison. A bactericidal antibody response was demonstrable within a week after the primary immunization against capsulated as well as non-capsulated bacteria. After the hyperimmunization course the BC titres against noncapsulated bacteria were about 1:4,000, while the BC titres against the capsulated - type a and b-strains reached levels of no less than 1:30,000. Samples of separated hyperimmune sera showed BC activity in IgM- as well as IgG-containing fractions, whereas the BC activity of primary response samples was demonstrable mainly in IgM-containing fractions. The BC effect on non-capsulated bacteria of primary response samples were more heat labile than those obtained after hyperimmunization. In immune sera a crossreactive BC activity was demonstrable on non-capsulated bacteria, the titre being 32-64-fold lower than that of the homologous activity.