IMMUNITY IN PNEUMOCOCCAL INFECTIONS 33 



By treating animals with repeated injections of aggressin, great 

 protective power is produced. An anti-aggressin is developed, by which 

 phagocytosis is enormously stimulated. Thus, after three inoculations 

 of an actively " aggressive " pleural exudate at intervals of five days, a 

 rabbit remained alive with a dose of pneumococcus emulsion which 

 killed another in 18 hours. Passive immunisation with the serum of 

 rabbits treated with aggressin seemed also to protect against pneu- 

 mococcal infection. 



In the mechanism by which the different anti-bodies act in the 

 production of immunity against pneumococcus, it will be seen that the 

 great immunity agent is phagocytosis, in which the leucocyte, in itself 

 a passive factor, is made capable of rendering great service to the 

 individual by engulfing the pneumococci which have been massed 

 into suitable clumps by the agglutinin, and rendered suitable pabulum 

 by the opsonin, while the leucocyte itself is excited to labour by the 

 stimulin in the serum. Aggressin also seems to be at the same time 

 capable of exciting or paralysing phagocytosis, according to whether 

 inoculated before or after infection. 



(397) 



