284 Chapter IX 



destruction of the micro-organisms in the vaccinated animals, though 

 in the normal animals this digestion is very imperfect. 



The acquisition of immunity against micro-organisms is, therefore, 

 due not only to the change from negative to positive chemiotaxis, but 

 also to the perfecting of the phagocytic and digestive powers of the 

 leucocytes — a general superactivity and adaptation of the phagocytic 

 reaction of the immunised animal is produced. This conclusion, 

 based upon a large number of well-established facts and in complete 

 harmony with the whole of the data at our disposal concerning 

 acquired immunity, has been attacked by Denys and Leclef ^ in their 

 work on the streptococcus. They base their opposition upon experi- 

 ments made in vitro on the action of serums and leucocytes on this 

 micro-organism. They have compared the bactericidal pow^er of 

 mixtures of the serums of normal and of vaccinated rabbits with 

 leucocytes isolated from exudations from these two groups of 

 animals. The leucocytes, whether derived from normal or from 

 vaccinated rabbits, when mixed with normal serum were equally 

 incapable of ingesting and destroying the streptococci. When mixed 

 Avith blood serum from vaccinated rabbits, however, the two kinds of 

 leucocytes exhibited a very marked phagocytic reaction. Denys and 

 Leclef conclude from this that phagocytosis, although an important 

 factor in immunity, plays merely a secondaiy part and is dependent 

 on the humoral properties. The experiments and views of these 

 two observers have been generally received by the partisans of 

 the bactericidal theory of the body fluids as an actual proof of 

 this theory. We cannot agree. Researches extending over a long 

 period have shown us that the study of phagocytosis in vitro can 

 give only a very inexact and imperfect idea of the course of the 

 phenomena in the living animal. Usually the leucocytes taken from 

 the exudations, although amoeboid, no longer fulfil their phagocytic 

 functions at a time when in the animal they w^ould ingest micro- 

 organisms with the greatest rapidity. As a general rule, existence 

 outside the living body w eakens them very considerably. But in some 

 cases, rare it is true, the leucocytes although inactive in the animal 

 [299] exhibit intense phagocytosis when introduced into a hanging drop of 

 fluid from an exudation or even of urine. In any case it is very 

 hazardous to infer from phenomena which appear under these 

 artificial conditions what takes place in the living animal. The value 

 of the experiments of Denys and Leclef is still further marred by 

 ^ La Cellule^ Lierre et Louvain, 1S95, t. xi, p. 177. 



