25 



cytosis was strikingly different. The cocci which had been sensitized in 

 normal serum were not ingested by the leucocytes, while those that had 

 been sensitized in immune serum were taken up freely. That is, the 

 serum has acquired something in the process of immunization by virtue of 

 which it is capable of so changing the virulent streptococci that they 

 become susceptible to phagocytosis. The results of the count are shown 

 in Table 14. 



TABLE 14. 



PHAGOCYTOSIS OF VIRULENT STREPTOCOCCI SENSITIZED IN IMMUNE RABBIT SERUM. 



1. Washed rabbit leucocytes + normal rabbit serum + streptococcus 



"Laura" 0.4 



2. Rabbit leucocytes suspended in immune rabbit serum y 2 hour, 



washed in NaCl solution, resuspended in normal serum + 

 streptococcus "Laura" 0.0 



3. Washed rabbit leucocytes in NaCl solution + streptococcus "Laura" 



sensitized in normal rabbit serum 0.6 



4. Washed rabbit leucocytes in NaCl solution + streptococcus "Laura" 



sensitized in immune rabbit serum 8.0 



The destruction of virulent streptococci by rabbit leucocytes in im- 

 mune serum can also be shown by means of plate cultures. For this 

 purpose 2 tubes were prepared containing (a) I c. c. defibrinated nor- 

 mal blood + .2 c. c. of a thick suspension of rabbit leucocytes, and (b) 

 i c. c. defibrinated immune blood -(- .2 c. c. of the same suspension of 

 leucocytes. Each tube was inoculated with one loopful of a virulent 

 streptococcus culture, incubated at 36 C. and plates made at intervals 

 with 2 loopfuls of blood from each tube. The plates were incubated for 

 24 hours and the colonies carefully estimated with the following result: 



Strepto- Colonies in Agar plates, 



cocci. Immeil. 2 hours. 5 hours. 



381 Leucocytes in normal blood 480 1200 SOOO 



381 Leucocytes in immune blood 460 300 5000 



In the tube containing immune blood + leucocytes the number of 

 organisms decreased during the first two hours but then began to in- 

 crease rapidly, while in the other tube they increased rapidly from the 

 beginning of incubation. The immune serum alone has no destructive 

 action on these organisms but acts as a culture medium for them. 



The following experiment was now made to determine whether the 

 opsonin of the immune serum can withstand a higher degree of heat 

 than that of normal serum. 



Experiment. .2 c. c. of normal serum and .2 c. c. of immune serum 

 in two small test tubes were heated to 60 C. for y 2 hour. To each 



