578 MICRO-ORGANISMS AND DISEASE [chap. 



be immunised against a particular toxin, and that its blood 

 and blood-serum thereby acquire a proportionate specific 

 antitoxic potency. 



Antitoxic diphtheritic blood-serum does not act anti- 

 microbically or germicidally in vitro, for, as ^^^right has 

 shown, antitoxic blood-serum of a diphtheria-immunised 

 horse forms a good artificial medium for the growth of 

 the diphtheria bacilli. 



This production of acquired or active immunity by 

 toxin, is apparently not the same as is created in the animal 

 body under natural conditions, that is when the animal body 

 acquires immunity against a particular infectious disease by 

 a previous attack of the disease. In the natural condition 

 when the animal body is subject to an attack the specific 

 microbe, having found entrance, lives and multiplies within 

 the infected body and causes the particular disease, and 

 after the body recovers and the microbes again disappear it 

 is found some time afterwards that it has acquired the 

 power to resist a new infection with the microbe, or if this 

 be injected in an otherwise sufficient dose the animal and 

 its tissues resist it, the microbes cannot now Hve in such a 

 tissue or such an animal body, they degenerate and die 

 and produce no disease. Evidently during the first attack 

 something was formed in the animal which after the disease 

 has passed off is present in the blood and tissues and which 

 acts inimically, germicidally against the same microbe. This 

 germicidal substance does not appear immediately on re- 

 covery (in Fraenkel's experiments on diphtheria in the 

 guinea-pig it requires two to three weeks for its appearance) ; 

 the same holds good for pneumonia, for cholera, typhoid, 

 and others ; further this germicidal or immunising action of 

 the blood and tissues does not in all eases last for an 



