xxi] PATHOGENIC ORGANISMS 573 



acquired, the sooner the point of acquired immunity is 

 reached, or in other words the greater or more virulent the 

 subsequent dose that the animal can bear. By repeated 

 injections of gradually increasing doses a high degree of 

 resistance is ultimately reached. While by the method of 

 conferring this acquired or active immunity against a specific 

 disease, used by Behring, Behring and Kitasato, Klemperer, 

 Roux, R. Pfeiffer, and others, the animal is allowed to recover 

 from the previous injection before a further injection of the 

 increased dose is administered, Loffler shows {Centralblatt 

 f. Bakt. und Parasit, February, 1896) that as regards the 

 typhoid bacillus, by injections of small doses of virulent 

 bacilli administered in very short intervals of a few hours, 

 immunity can be acquired already in a few days. 



The fact that a specific immunity can be thus acquired 

 by injections of specific toxin, that is to say, by the repeated 

 injections of the pure toxin elaborated by a particular 

 microbe, and separated from the latter by filtration, e.g. 

 diphtheria toxin, tetanus toxin, typhoid toxin, &c., proves 

 conclusively that the condition of this immunity cannot 

 be due to a phagocytic action of the leucocytes ; no 

 microbes being used for the injections, there are no 

 microbes to be swallowed up and destroyed. 



Now, the most striking fact that was first demonstrated 

 by Behring and his co-workers is this : the blood or blood- 

 serum of an animal actively immunised, or, generally speak- 

 ing, of the animal body which has acquired immunity in one 

 way or another against a particular infectious disease, pos- 

 sesses a definite and measurable power to confer immunity, 

 passive immunity, against that particular disease if injected 

 into a normal animal ; more than that : it is capable of 

 modifying or even completely neutralising — curing — the 



