The intrinsic affinity constant (K) of anticapsular antibody to oligosaccharides of Haemophilus influenzae type b.

Antibody to the polyribosylribitol phosphate (PRP) capsular polysaccharide of Haemophilus influenzae type b is crucial to host defense. Affinities of antibody elicited by vaccination with PRP and PRP-diphtheria toxoid conjugate were determined using oligosaccharides (OS) from PRP. The affinities of antibody induced by vaccination with PRP to OS of three and four repeat units were similar but greater than the affinity to the two-unit OS. NaBH4 reduction of the three-unit OS did not alter the binding affinity, indicating that the reducing end of the OS did not participate in antibody binding. Over the range of OS concentrations tested, antibody affinity appeared to be homogeneous. Antibody concentration could be determined from binding experiments independently from affinity. Whole serum had 8- to 40-fold less antibody detected by binding analysis than by RIA, but the antibody concentration of an IgG fraction measured by the two methods agreed within a factor of two. We could not account for the discrepancy in concentrations found with whole serum by the presence of IgM or IgA antibody. The average affinity of antibodies of 10 adults vaccinated with PRP was similar to that of antibodies elicited in 14 adults vaccinated with a PRP-diphtheria toxoid conjugate (10.7 x 10(5) vs 7.6 x 10(5) liter/mol, respectively, p greater than 0.05). We conclude that the intrinsic affinity of antibody after vaccination with PRP is low and is not different from that of antibody elicited by PRP diphtheria toxoid conjugate.