108 STUDIES IN IMMUNITY. 



consider the relative sensitivity of the rabbit and the guinea-pig 

 to the streptococcus. 



If something near the minimal lethal dose of a virulent culture 

 has been injected into the peritoneal cavity of a guinea-pig a prog- 

 nosis may generally be given by the presence or absence of strepto- 

 cocci with an areola outside the cells. If, for example, four hours 

 after infection the exudate contains a small number of extracellular 

 streptococci with areola and a great many leucocytes, the fate of the 

 animal is doubtful. If the number of such bacteria is very small, 

 so that they may be found on a slide only with difficulty, they fre- 

 quently become the prey of exceptionally active phagocytes. But 

 if their number is at all considerable we may feel sure that they will 

 soon multiply unrestrictedly. 



The issue of streptococcus peritonitis is thus soon indicated and 

 the outcome of the conflict between cells and bacteria is clear very early. 



Mechanism of cure. We may review the evolution of strepto- 

 coccus peritonitis in the guinea-pig by saying that the cure of an 

 animal is due to phagocytosis. The existence in the bacteria of a 

 negative chemiotactic influence preventing the accomplishment of 

 phagocytosis, excludes a cure. Since the streptococci that in the 

 beginning have been able to protect themselves from phagocytes 

 multiply outside the cells without changes of morphology or color 

 reaction or any diminution in their virulence, there is nothing that 

 leads us to suppose that when animals withstand an infection there are 

 any factors in their cure other than phagocytosis. It may be simply 

 mentioned here that virulent streptococci taken up by the phago- 

 cytes of the guinea-pig are absorbed without losing their activity. 



The injection of a small amount of guinea-pig peritoneal exudate 

 containing no free streptococci, but with leucocytes containing 

 engulfed streptococci, into a rabbit within four hours after removal, 

 kills as a rule in 24 hours. 



We have already said that a guinea-pig that has received a dose 

 of streptococci small enough to allow the bacteria to be entirely 

 engulfed by phagocytes recovers without any further trouble. As 

 a general rule, indeed, when complete phagocytosis takes place the 

 animal is at once out of danger and soon recovers. Within two or 

 three days the exudate ceases to contain living bacteria. In a few 

 rare instances an animal that has lived several days without showing 



