248 A MANUAL OF BACTERIOLOGY 



and serous exudation occur, and thus the germicidal 

 action of the body fluids and tissues might be exerted 

 in vivo. On the other hand, the action may be one of 

 stimulating the leucocytes or rendering the bacteria more 

 phagocytosable (as will be referred to later, p. 255) 

 rather than a direct germicidal one. 



Thus Kanthack and Hardy found that the coarsely 

 granular oxyphile leucocytes in the frog are first attracted 

 to the site of a bacterial invasion, there discharge their 

 oxyphile granules, the bacteria then show signs of de- 

 generation, and polymorphonuclear leucocytes and other 

 " phagocytic " cells now approach and ingest the degene- 

 rate bacteria. The observations, however, do not seem to 

 have been confirmed. Wooldridge also protected animals 

 from anthrax by injections of " tissue fibrinogen " (nucleo- 

 protein). For some micro-organisms a bacteriolytic 

 mechanism exists, the amboceptor-complement complex, 

 whereby they may be digested and eliminated. 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 3 so ably supported by Metchnikoff. 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 assemble at the injured spot. Similarly, in many 

 instances the leucocytes rapidly congregate at the seat of 



