50 IMMUNITY 



by him that the educated leucocytes neutralize the poison by 

 their secretions. 



In the case of anthrax infection, animals infected with virulent 

 cultures of this organism quickly succumb, without exhibiting 

 any leucocytosis (negative chemo taxis). 



If the animal has been previously immunized with attenuated 

 culture the injection of a virulent culture is followed by an enor- 

 mous outpouring of leucocytes at the site (positive chemo taxis), 

 while if the site of the inoculation in the non-immune animal is 

 examined, only a few leucocytes, and some clear serum will be 

 found. 



Toxins, if injected, cause a negative chemotaxis. If tetanus 

 spores are injected into an animal, together with some toxin, the 

 animal rapidly succumbs to tetanus, without evincing any leuco- 

 cytosis. If the spores are washed free from toxin, and injected, 

 active leucocytosis occurs and the animal survives. 



A mixed infection of a highly virulent culture, and a non- 

 virulent one, often hastens the action of the virulent one. It 

 is supposed that the non-virulent bacteria engage the leucocytes, 

 so that these cells cannot cope with the virulent ones. 



Phagocytosis thus plays an important part in a protective role 

 in natural immunity, but no satisfactory theory has yet been 

 offered in explanation of the protective process in acquired 

 immunity, at least against toxins and other soluble and unorgan- 

 ized poisons. 



In order to meet the criticisms arising after Ehrlich's theories. 

 Metchnikoff added to his theory by stating that complement 

 and-anti-body are enzymic substances derived from phagocytes. 



The cellulo-humoral theory claims the attention of most bac- 

 teriologists, as the probable explanation of the phenomena of 

 immunity. It is certain that cells, either sessile or mobile, and 

 fluids, are important means of internal defense. In order that ' 

 this theory may be comprehended, certain well-known properties 

 on formal and artificially immunized serum must be understood. 



