Effect of immunization with Freund's adjuvant and pneumolysin on histologic features of pneumococcal infection in the rat lung in vivo.

Immunization with Freund's adjuvant and pneumolysin and stimulation with Freund's adjuvant alone both reduced the severity of the pneumonia caused by injections of bacteria into the apical lobe bronchi of rats. Neither protocol influenced the incidence of pneumococcal bacteremia. Illness sufficiently severe to require sacrifice was delayed from 2.8 days in nonimmunized animals to 5.7 days in those immunized with Freund's adjuvant and pneumolysin (P < 0.05) and 4.5 days in those stimulated with Freund's adjuvant alone (P, not significant).