202 A MANUAL OF BACTERIOLOGY 



protein). For some micro-organisms a bacteriolytic 

 mechanism exists, the amboceptor- complement complex, 

 whereby they may be digested and got rid of. Thus 

 normal serum has a marked bacteriolytic action on B. 

 typhosus and B. coli. In many cases, however, e.g. for 

 staphylococci, such a bacteriolytic mechanism does not 

 naturally exist, but may be evoked as a result of infection. 



The hypothesis which ascribes immunity to the germi- 

 cidal and bacteriolytic action of substances in the fluids 

 of the body has been termed the " humoral theory." 



Another important theory of immunity is the doctrine 

 of phagocytosis, so ably supported by MetchnikofL This 

 is the " cellular " theory of immunity. It has as its basis 

 the following fundamental facts : Firstly, the leucocytes 

 in the circulating blood ingest and destroy any foreign 

 particles present therein ; secondly, an injury to the 

 tissues is immediately followed by an inflammatory reac- 

 tion, in which the leucocytes emigrate from the vessels 

 and congregate at the injured spot. Similarly, in many 

 instances the leucocytes rapidly congregate at the seat of 

 a bacterial infection, and approach and engulf the bacteria 

 in the same manner as they do other foreign particles, and 

 so rid the body of the unwelcome guests (Plate I., a and b). 



The migration of the leucocytes towards the scene of 

 action is explained by MetcrmikofT on the hypothesis that 

 the chemical substances elaborated by the bacteria attract 

 the latter and exert what he termed " positive chemo- 

 taxis." In this case the bacteria are removed by the 

 leucocytes, and general infection and death do not occur. 

 But, unfortunately, in other cases the bacterial chemical 

 products repel, or perhaps it is more correct to say do not 

 attract, the leucocytes, and " negative chemotaxis " occurs, 

 so that the bacteria are free to grow and multiply, and 

 general infection ensues. Positive and negative chemo- 

 taxis can be shown to occur by a simple experiment. If 



