RELATION OF LEUKOCYTES TO IMMUNITY 297 



bacilli was demonstrated in such naturally resistant animals as dogs 

 and chickens, while almost no cell ingestion occurred in delicately 

 susceptible animals like guinea pigs and rabbits. Rats, on the other 

 hand, more resistant to anthrax than guinea pigs, less so than dogs, 

 showed a degree of phagocytosis intermediate between that observed 

 in the cases of the other animals mentioned above. And yet, in these 

 more susceptible animals, the normal bactericidal action of the blood 

 upon anthrax bacilli, though never extreme, was often more marked 

 than that of the naturally immune animals mentioned above. 



It is well known, for instance, that the serum of dogs possesses 

 almost no bactericidal properties for anthrax bacilli, 1 although the 

 animals are highly resistant to this infection, while the serum of 

 rabbits is probably more strongly bactericidal for these bacilli than 

 the serum of most other animals, and yet rabbits are extremely 

 susceptible. That the lack of bactericidal powers of the serum is 

 not always a sign of susceptibility on the part of the animal was 

 shown as early as 1889 by Lubarsch. (We must remember, how- 

 ever, 'that lack of bactericidal power does not necessarily mean lack 

 of sensitizer. For bacteria may be sensitized without being killed 

 extracellularly as can be shown by the alexin-fixation reaction.) 



The study of anthrax infections was a peculiarly fortunate 

 choice of subject, since in this bacillus resistance to serum lysis is 

 especially well marked and phagocytosis seems indeed to be the chief 

 mode of bacterial destruction. Studies analogous to those originally 

 made with anthrax, however, were subsequently carried on with 

 streptococci, pneumococci, and staphylococci chiefly by Bordet, 2 

 Marchand, 3 and others, and results coinciding with those of Metchni- 

 koff were obtained. In every case naturally resistant animals 

 showed marked phagocytosis, and susceptible ones failed to show it 

 to the same degree. It is a strong support of the same opinions, too, 

 that Marchand' s studies, later extensively confirmed, showed that 

 the more virulent and invading strains of streptococci, the less active 

 is the phagocytosis a converse, but equally conclusive, observation. 



Further support for this point of view is manifold and cannot be 

 considered with anything like completeness. We may refer briefly, 

 however, to the experiments of Vaillard, Vincent, and Rouget 4 with 

 tetanus, and those of Leclainche and Yallee 5 with symptomatic 

 anthrax, because they are especially valuable in illustrating the 

 importance of phagocytosis in another class of infection. The pois- 

 ons of these micro-organisms are extremely toxic for rabbits, and if 



1 Petterson. Centralbl. f. Bakt., 1, 39, 1905. 



2 Bordet. A nnl de I'Inst. Past., Vol. 11, 1897. 



3 Marchand. Archiv. de med. Exp., Vol. 10, 1898. 



4 Vaillard, Vincent, and Rouget. Ann. de I'Inst. Past., Vols. 5, 6, 1891- 

 1892. 



5 Leclainche and Vallee. Ann. de I'Inst. Past., Vol. 14, 1900. 



