10 Introduction 



in any other part of the body, than are the large domestic mammals 

 such as the ox and the horse. And yet these latter are very 

 liable to epizootic anthrax, whilst the rodents mentioned are seldom, 

 if ever, attacked by spontaneous anthrax. This apparent immunity 

 in no way depends on the existence of a true immunity of the 

 organism, but solely on the conditions under which mice and guinea- 

 pigs live. 



\Ve shall therefore in this volume treat only of the phenomena of 

 organic immunity in living beings, and the problem, even restricted 

 within these limits, still appears sufficiently complex. With the 

 object of rendering its study as easy as possible, it will be useful to 

 commence by giving an account of the phenomena of immunity in the 

 lowest organisms. 



Immunity against infective diseases should be understood as the 

 group of phenomena in virtue of which an organism is able to resist 

 the attack of the micro-organisms that produce these diseases. It 

 is impossible, at present, to give a more precise definition, and 

 useless to insist upon it. Some have thought it necessary to dis- 

 tinguish between immunity properly so called, that is to say a 

 permanent refractory state, and "resistance," or a very transient 

 property of opposing the invasion of certain infective micro-organisms. 

 It is not possible to maintain this distinction, for in reality the limits 

 between these two groups of phenomena are far from being constant. 

 [11] Immunity may be inborn or acquired. The former is always 

 natural, that is to say, independent of the direct intervention of 

 human art. Acquired immunity is also often natural, from the fact 

 that it is established as the result of the spontaneous cure of an 

 infective disease. But in a great number of cases acquired immunity 

 may be the result of direct human intervention as in the practice 

 of vaccination. 



For a long time all the phenomena of immunity against infective 

 diseases were collected into a single group. Later, it was recognised, 

 as the result of the demonstrations summarised at the beginning of 

 this chapter, that it is necessary to distinguish sharply between 

 immunity against the pathogenic micro-organisms themselves and 

 that against microbial poisons. Hence the idea of antimicrobial 

 and antitoxic immunities. In the course of this work this essential 

 distinction must always be borne carefully in mind. 



