Anticapsular antibody requirements for protection against experimental Haemophilus influenzae type b bacteremia after splenectomy.

Although asplenic individuals are at higher risk for infection with encapsulated bacteria, it is not known whether they require a higher concentration of anticapsular antibody than normal individuals do for protection against invasive disease caused by Haemophilus influenzae type b. At 21 days of age, rats were passively immunized with human hyperimmune serum globulin against H. influenzae b polysaccharide (or saline) after recovery from splenectomy or a sham operation. Starting at 18 h after immunization, rats received three intranasal inoculations of 10(7) CFU of H. influenzae b over the next 24 h. Of sham-operated rats given 0.75 or 3.0 micrograms of anticapsular antibody, 91 or 96%, respectively, were protected from bacteremia, whereas only 59 and 67% of similarly treated asplenic rats were protected (P less than 0.004, control versus asplenic rats). A 12-microgram antibody dose resulted in the complete protection of both groups. The magnitude of bacteria was significantly higher in the asplenic group at each dose of antibody. Thus, asplenic hosts may require a higher concentration of anticapsular antibody than normal individuals do for protection against invasive H. influenzae b disease.