122 IMMUNITY. 



particular substance, and at the time of the second infection 

 died for want of it. This theory is no longer tenable, as we 

 have shown that even an immune will succumb to an over- 

 whelming infection, so that a vital substance can have noth- 

 ing to do with the production of immunity. 



Retention theory : Chauveau suggested that the bacteria 

 in the course of their development elaborate some metabolic 

 product which accumulates in the body and is retained. This 

 product is inimical to the growth of that particular organism, 

 and a second infection cannot, therefore, occur. 



Under normal conditions the tissues and fluids of the body 

 are constantly undergoing changes, and it is extremely 

 improbable that any metabolic product of this kind could be 

 retained unchanged for an indefinite number of years. Fur- 

 thermore, it is also improbable that an individual who has 

 suffered from a number of different infections could keep 

 stored up in his body the products of the various organisms 

 causing these infections in sufficient quantity to prevent their 

 development at a subsequent infection. 



Germicidal action of the body-juices : It has repeatedly been 

 demonstrated that freshly drawn blood, aqueous humor, 

 ascitic fluid, and the lymph from the dorsal lymph-sac of the 

 frog, possess a decided germicidal power. This power is 

 positive, but is limited as to the number of bacteria which 

 can be destroyed by a given quantity of serum. Heat 

 promptly destroys this power. The theory is therefore 

 untenable. 



Phagocytosis : This theory has already been fully discussed. 



Antitoxins : When it was discovered that bacteria elabo- 

 rated toxins, the conclusion was arrived at that immunity to 

 these toxins must be due to an antagonistic substance which 

 neutralizes the toxins. This substance was called antitoxin. 

 These antitoxins are probably one of the phenomena of immu- 

 nity rather than the cause of it. They will be discussed 

 fully in the next chapter. 



Ehrlich's lateral-chain theory : Ehrlich believes that the 

 various cells of the body contain a nucleus to which are 

 attached by means of what he terms " side-chains " certain 

 atom groups. These side-chains are concerned in the nutri- 



