PATHOGENICITY FOR EXPERIMENTAL ANIMALS 209 



addition of antirough pneumococcic serum to normal rabbit serum 

 and cells resulted in increased phagocytosis of the strains that 

 had been partially inhibited by the C Fraction ; and antirough 

 pneumococcic rabbit serum protected mice against one hundred or 

 more minimal lethal doses of the rabbit-virulent strain, provided 

 the organisms were injected by the intravenous route. When anti- 

 rough serum was absorbed with the C Fraction, the mouse protec- 

 tive property was removed. In the summary and general conclu- 

 sions, the authors stated: 



Since there is no evidence for the occurrence of type specific antibody 

 in the normal rabbit and since, as we have shown, the Pneumococcus 

 Type III whether avirulent or virulent is not removed from the blood 

 stream or destroyed when the capsule is intact, the following factors 

 which have been revealed in the course of our work appear to represent 

 certain essential components, if not the complete mechanism, upon 

 which the natural immunity of the rabbit against this organism de- 

 pends, (a) The elevation of the body temperature after intravenous in- 

 fection to 41 °C. or thereabouts and its maintenance for varying peri- 

 ods, (b) The ability of the phagocytic cells, both fixed and mobile, to 

 attack any cocci which have become vulnerable through the deteriora- 

 tion of capsular integrity, (c) The adjuvant effect of an antibody, re- 

 acting with the somatic C carbohydrate, which enhances the phagocyto- 

 sis of such organisms as no longer possess a completely intact envelope. 



Conversely, the varying degrees of virulence for rabbits observed 

 among Pneumococcus Type III strains are based upon: (a) differences 

 in the ability of the organisms to multiply at the elevated temperatures 

 encountered in the infected host. Strains markedly susceptible to the 

 harmful influence of this factor fail to induce a generalized fatal infec- 

 tion. Not all "thermo-resistant" strains are highly virulent, however, 

 and these may contrast sharply with regard to (b) size of the capsule 

 and the ease with which it is impaired or completely lost. The capsules 

 must be maintained intact for a sufficient time until multiplication of the 

 organisms can proceed to such a degree that death of the host results. 

 Avirulent strains even when capable of growth at 41 °C. appear to be 

 unable to satisfy this requirement. 



The differences in virulence of various strains apparently condi- 

 tioned by these factors are not limited solely to the case of the rabbit, 

 since for at least two strains similar differences in virulence have been 



