Immunology of the infant rat experimental model of Haemophilus influenzae type b meningitis.

The age-related acquisition of serum anticapsular and bactericidal antibodies to Haemophilus influenzae type b observed in rats was similar to that of humans. The antigenic source for this "natural" immunity was not identified since neither pharyngeal infection with H. influenzae b nor enteric colonization by cross-reacting bacteria was detected. Infant rats surviving H. influenzae b bacteremia failed to respond immunologically to the capsular polysaccharide. However, surviving rats demonstrated no impairment of immune responsiveness to this antigen after subsequent immunization with live bacteria in adulthood. In passive protection experiments, antibodies directed against the type b capsular polysaccharide represented the major protective specificity. However, a small protective effect of antibodies to noncapsular antigens also appeared to have been demonstrated.