HOST RESPONSE TO ANTIGENIC ACTION 429 



ologous species. While in the case of the rabbit and mouse no 

 phagocytosis was observed when virulent pneumococci were used 

 in the test, the destruction of avirulent strains seemed to parallel 

 the resistance toward the particular strain possessed by the ani- 

 mal whose serum was tested. While Dold (1911) 323 could demon- 

 strate no substances antagonistic to Pneumococcus in the serum, 

 plasma, or whole blood of normal mice and rabbits, by treating 

 rabbit leucocytes by the method of Schneider, extracts were ob- 

 tained which Dold claimed had definite killing power for pneu- 

 mococci. 



Robertson and Sia (1923) 1144 devised an accurate method for 

 demonstrating growth-inhibitory and bactericidal action on Pneu- 

 mococcus of normal serum-leucocyte mixtures. The ingredients of 

 the mixture were added in known quantities and mechanically agi- 

 tated by rotation and oscillation. A combination of serum and 

 leucocytes from resistant animals (cats and dogs) exerted not 

 only a growth-inhibiting but also a bactericidal action on pneu- 

 mococci. The serum-leucocyte mixtures of susceptible animals 

 (rabbits and guinea pigs) showed no inhibitory effect. In subse- 

 quent communications, Robertson and Sia 1145 " 7 substantiated their 

 earlier results. For example, the growth of pneumococci possess- 

 ing low virulence for the cat was found to be markedly inhibited 

 in mixtures of cat serum and cat leucocytes, since ten thousand 

 times the number of pneumococci ordinarily sufficient to kill a 

 mouse failed to infect after being exposed for twenty-four hours 

 in the cat serum-leucocyte mixtures. Furthermore, virulent strains 

 sensitized by contact with serum of animals resistant to Pneumo- 

 coccus were actively phagocyted, not only by the homologous leu- 

 cocytes, but also by the leucocytes of other resistant animals and 

 of susceptible animals. However, pneumococci exposed to the ac- 

 tion of serum from susceptible animals were not taken up by leu- 

 cocytes of either the resistant or susceptible animals. The serum 

 of all the resistant animals tested — dog, cat, sheep, pig, and horse 



