Mechanism of immunity against micro-organisms 181 



had shown that the virus of fowl cholera, which in the guinea-pig 

 sets up a mild affection and gives rise to the formation of abscesses, 

 retains its virulence for a considerable time in the pus of these ab- 

 scesses. When he injected rabbits with a small quantity of guinea- 

 pig's pus developed at the point of inoculation of the cocco-bacillus 

 of fowl cholera, the animals succumbed to a generalised and rapid 

 infection. The conviction has since been arrived at that, in the 

 guinea-pig, these micro-organisms readily become the prey of the 

 leucocytes that are present in the exudations. 



The rule, therefore, is general that in animals endowed with 

 natural immunity the phagocytes seize and ingest even living micro- 

 organisms that have retained their initial virulence. 



Once within the phagocytes, the micro-organisms are surrounded 

 by a clear fluid, which accumulates in vacuoles, or they are lodged 

 directly in the protoplasm. In both cases the micro-organisms are 

 subjected to a digestive action which usually dissolves them com- 

 pletely. It is not always easy to form an idea of the conditions under 

 which the intracellular digestion takes place. At first 1 I used a weak 

 solution of vesuvin for the purpose of gaining some idea as to the 

 condition of the micro-organisms that have been ingested by the leu- 

 cocytes and demonstrated that the living bacteria remain unstained 

 in this solution, whilst the dead bacteria take on a somewhat deep 

 brown stain. Thanks to this reaction I was able to furnish one of the 

 proofs of the fact that in immunised animals ingested bacteria are 

 killed inside the phagocytes. The use of Ehrlich's neutral red (Neu- 

 tralrotJi) gives us further valuable indications. This colour, quite 

 innocuous for living elements, is an excellent indicator of acid or 

 alkaline reaction. Plato 2 , in Breslau, has carried out numerous [192] 

 researches on the staining of micro-organisms by a weak aqueous 

 solution (l/ )of this substance. He has shown that "free" micro- 

 organisms remain alive in this solution without taking on any tinge 

 of colour. On the other hand, the same micro-organisms, when 

 ingested by the phagocytes, are stained brownish-red. Most of these 

 stained organisms no longer exhibit any sign of vitality ; but amongst 

 those within the phagocytes are some which, in spite of being deeply 

 stained, are certainly alive. Plato insists on the fact that ingested 

 micro-organisms remain stained as long as the phagocytes are 

 alive, for, shortly after the death of these cells, decoloration of the 



1 Ann.de I'lnst. Pasteur, Paris, 1887, t. I, p. 325. 



2 Areh.f. mikr. Anat., Bonn, 1900, Bd. LVI, S. 868. 



