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or neutralising its toxins, or an insusceptibility to the action of 

 the toxine. 



Different animals have different degrees of resistance or 

 non-susceptibility to toxic bodies — a fact so far incapable of 

 explanation. We must take this natural resistance for granted, 

 and there is no evidence that for each case there is an anti-toxic 

 body present which protects ; the serum of a fowl, for instance, 

 does not protect another animal from tetanus, though the serum 

 of a less susceptible animal in which a resistance equal to that 

 of the towl has been artifically developed does possess anti-toxic 

 powers. The resistance evidently lies in the tissues. 



With regard to the natural bactericidal powers, the 

 powers seem to reside in phagocytosis and in the action of the 

 serum, the latter deriving its virtue from substances derived 

 from various glands, spleen, lymphatic glands, the gums, etc., 

 and termed alexines. 



Having now discussed the various methods of conferring 

 acquired immunity, there remain to be considered the hypo- 

 theses which have been built up to explain the processes in the 

 animal body by which the immunity is gradually developed. 

 None of them, however, are capable of satisfying every aspect 

 of the case. 



(1.) Pasteur's theory of exhaustion — in other words, it is 

 assumed that there are certain substances in the body of a living 

 animal necessary for the existence of a particular germ, and 

 that this is used up by the germs which then die out. This is 

 of course easily disproved by the discovery of passive immunity 

 conferred by the injection of the serum of an immunised animal, 

 i.e., a small quantity of serum in which the pabulum has been 

 exhausted cannot lead to its exhaustion in the serum of another 

 animal into which it is introduced. 



(2.) Theory of retention which supposes that the toxins 

 elaborated by the bacilli gradually kill them — as happens in cul- 

 tures in test tubes : this does not explain how it is that acquired 

 immunity may last for years, as it is unreasonable to suppose 

 that these toxins are retained in the system during that time. 



(3.) Phagocytosis — Metchnikoff's wonderful theory, which 

 credits the lencocytes with bactericidal powers. This theory has 

 demonstrated one of the chief methods possessed by the body in 

 dealing with invading organisms, but does not explain the con- 

 ferring of immunity by means of serum injections or chemical 

 vaccines. 



