Correlation between antibody affinity and serum bactericidal activity in infants.

Nearly one-half of infants immunized with Haemophilus influenzae b capsular polysaccharide (polyribosylribitol phosphate; PRP)-protein conjugate produce low-affinity antibody. To test the hypothesis that antibody affinity is linked to biologic function, sera were obtained before and 1 month after immunization of 18-month-old infants with PRP-diphtheria toxoid conjugate vaccine. Correlation was attempted of anti-PRP affinity, concentrations of anti-PRP, and anti-outer membrane proteins and of immunoglobulin isotype with bactericidal activity. Nine subjects produced anti-PRP of low affinity (K less than 10(4) l/mol), and 11 had higher affinity antibodies (average K, 2.8 x 10(4) l/mol). By multiple regression analysis, antibody affinity was the only variable significantly related to the bactericidal activity of serum after immunization with the conjugate vaccine (r = .71; P = .04). Thus, serum anti-PRP from a substantial proportion of infants appeared functionally deficient in association with low-affinity antibody.