24 



satisfactory than those made from suspensions of leucocytes in serum. 

 Four-tenths of a cubic centimeter of each suspension of leucocytes in 

 blood was put into a number of small test tubes arid .2 c. c. of a fairly 

 thick suspension of virulent streptococci added to each tube. The tubes 

 were incubated at 36 C. for i hour, smears were made and the average 

 number of cocci ingested by each leucocyte was determined by counting 

 the cocci in 30 leucocytes. The results of such an experiment are shown 

 in Table 13. 



The Table shows that there is a larger number of cocci taken up 

 by the leucocytes in immune blood than by those in normal blood; that 

 the immune leucocyfes do not ingest appreciably more cocci than the 

 normal leucocytes and that the increase is most marked with that strain 

 of streptococcus which was used in immunizing the rabbit. 



Four other experiments with the bloods of different immune rabbits 

 gave practically the same results. The increased phagocytosis always 

 was most pronounced with that strain of streptococcus which had been 

 used in immunizing the rabbit whose defibrinated blood or serum was 

 used in the experiment. Several strains of streptococcus were used in 

 the immunization and all that were of medium virulence gave an im- 

 munity that could be demonstrated in the test tube. Rabbits were also 

 treated with a highly virulent streptococcus, but this organism was 

 scarcely taken up at all by rabbit leucocytes suspended in this or in other 

 immune sera. 



An experiment was now carried out to show how the immune serum 

 promoted phagocytosis of the virulent cocci. For this purpose normal 

 rabbit leucocytes were suspended in normal and some in immune rabbit 

 serum for */?, hour, then centrifugated out of the sera, washed twice in 

 NaCl solution and suspended in normal serum. To each tube was now 

 added the same amount of suspension of a virulent streptococcus, the 

 tubes were incubated for i hour, smears were made from each and the 

 degree of phagocytosis determined by counts. In neither tube was there 

 any phagocytosis, showing that the immune serum is not capable of 

 changing the leucocytes so that they will ingest virulent streptococci 

 which have not been treated with immune serum. A second pair of 

 tubes was carried through at the same time, using the same leucocytes, 

 streptococcus suspension and sera. In this instance virulent streptococci 

 were treated J/ hour with immune rabbit serum and some with normal 

 rabbit serum, then washed once in a large amount of NaCl solution, sus- 

 pended in NaCl solution and each lot added separately to a suspension 

 of normal rabbit leucocytes in NaCl solution. The tubes were in- 

 cubated i hour, smears were made as above and the degree of phagocy- 

 tosis determined by counts. In this pair of tubes the degree of phago- 



