180 IMMUNITY. 



tion, and vice versa, is no proof that this phenomenon is not a result 

 of inhibitory action in those cases in which it does occur. It should 

 be understood that agglutination is only one of various indications 

 that the body juices of immunized animals rob their homologous bac- 

 teria in part of their virulence. Why agglutination does not take 

 place in all instances we are not as yet able to determine, but when 

 it does occur it is an indication that the blood of the immunized ani- 

 mal has some detrimental effect upon the growth and virility of the 

 microorganism. It is true, as Metschnikoff has pointed out, that 

 agglutination is not uniformly observed, and that there are im- 

 munized animals which furnish sera in which the homologous bac- 

 teria grow quite normally, but this observation does not overthrow 

 the fact that agglutination is an evidence of the detrimental effect of 

 the sera of immunized animals on the bacteria to which such animals 

 have been immunized. 



In some instances the body juices of the immunized animal mani- 

 fest a bactericidal action on their homologous microorganisms, but 

 have no effect on their toxins. This was shown to be the case by 

 Metschnikoff in his experiment upon the coccobacillus of swine plague. 

 The blood sera of animals immunized to this bacterium protect 

 rabbits against infection, but furnish no protection against the toxin. 

 In this instance we have to do with purely antiinfectious phenomena. 

 The immunity secured is antibacterial and not antitoxic. The anti- 

 infectious properties of the body juices are not, in all instances at 

 least, strictly specific. The sera of certain immunized animals have 

 a bactericidal action not only on their homologous bacteria, but 

 sometimes on closely related microorganisms. Indeed in some in- 

 stances it is not necessary that the relationship between the bacteria 

 affected in like manner by these antiinfectious sera should be close. 

 Thus, as has already been stated in the chapter on agglutinins, the 

 blood serum of animals immunized to the cholera vibrio also aggluti- 

 nates several other vibrios, and it has been shown by Issaeff that the 

 blood serum of animals immunized to the vibrio of Metschnikoff has 

 also an antiinfectious action on the pneumococcus. It has also been 

 shown that normal serum may in some instances manifest marked 

 antiinfectious properties. For instance, one-tenth cubic centimeter 

 of the blood serum of a healthy man suffices to protect guinea-pigs 

 against cholera peritonitis, and it is well known that frequently the 

 blood serum of men, who have never had typhoid fever, agglutinates 

 readily the bacillus of this disease. Not only is this the case, but 

 the blood serum of such men may protect animals against the perito- 

 nitis normally induced in them by the typhoid bacillus. It follows 

 from these observations, which might easily be multiplied, that the 

 blood of normal animals frequently contains antiinfectious substances. 



The bacteriolytic effect is the most marked form of bactericidal 

 action that has been observed in the body juices of artificially im- 



