Age-related susceptibility to Haemophilus influenzae type b disease in rabbits.

Anti-type b antibodies were detected in newborns and in most 6- to 8-week-old rabbits, similar to those observed in humans. Culture of the pharynx and rectum of rabbits at varied ages failed to yield Haemophilus influenzae type b. Bacteria with cross-reacting antigens were observed in both the pharyngeal and rectal cultures from rabbits of varied ages. Rabbits without preexisting serum anti-type b antibodies were highly susceptible to septicemia and death after injection of small numbers of H. influenzae type b. Passively acquired antibody, whether observed in newborns or injected into animals without preexisting antibodies, conferred a high degree of protection against H. influenzae type b.