Immunity against micro-organisms 315 



recovery is always this complete ingestion " (p. 203). If the ingestion 

 is not general, the rabbit may die, although much later than the 

 control animal. 



The phases of the struggle between the animal organism, when 

 subjected to the influence of the protective serum, and the strepto- 

 coccus, recall Salimbeni's experiments on immunised horses. The 

 rabbit, in which phagocytosis could not take place at once owing to 

 the presence of too large a number of micro-organisms, exhibits first 

 a stage of free development of the streptococci, after which the 

 phagocytes begin to fulfil their antibacterial function. Here it is 

 especially the macrophages which act ; the microphages, although 

 present in fairly large numbers, are entirely inactive. This first stage 

 of phagocytic reaction is insufficient. It is followed by a period when [331 

 the streptococcus appears to gain the upper hand. Many small 

 chains, having escaped the phagocytes, multiply and give birth to 

 quite a new generation of micro-organisms. If a fresh impulse to 

 phagocytosis does not take place the animal dies from infection. 

 When, however, the protective serum has been of sufficient strength, 

 a new army of leucocytes arrives on the scene and these become 

 masters of the situation. Phagocytosis becomes complete and micro- 

 phages as well as macrophages devour a large number of streptococci. 



Bordet, who, through his previous investigations, was well ac- 

 quainted with the direct action of the protective serum on vibrios, 

 could find nothing resembling it taking any part in the struggle of the 

 organism of the animal treated with antistreptococcic serum against 

 the streptococcus. The most that he could find was that the strepto- 

 cocci which again begin to swarm in the exudation are smaller in size 

 than the normal streptococcus. It must be accepted, as indicated by 

 the most recent researches, that this micro-organism becomes per- 

 meated by the fixative substance of the specific serum. We know 

 already, however, that this fixation cannot deprive the micro-organisms 

 of their virulence. In any case, then, a large share in the process 

 must be attributed to the action of the phagocytes, stimulated by 

 the protective serum, in the struggle of the animal against the 

 streptococcus. 



Having considered this series of examples of immunity against 

 bacteria conferred by specific serums, we are in a position to form 

 some idea of the mechanism of this immunity. Before we come to any 

 general conclusion, it may be useful to glance at an example of this 

 so-called passive immunity against a micro-organism belonging to 



