48 STUDIES IN IMMUNITY. 



It is evident from this experiment that normal serum may be 

 made strongly bactericidal by the addition of a small amount of 

 preventive serum, as shown by the plate method. This fact may 

 also be demonstrated by adding a small drop of a suspension of 

 vibrios to a drop of the serum mixture, when it will be found that 

 the organisms are rapidly transformed into granules. Pfeiffer's 

 phenomenon, then, may be produced in vitro as we shall presently 

 consider more in detail. In the same way that a small amount of 

 preventive serum vaccinates animals and gives them a bactericidal 

 property against the vibrio, so it would seem to " vaccinate" normal 

 serum as evidenced by the intense bactericidal property which this 

 serum acquires. 



VII. THE SPECIFICITY OF THE PREVENTIVE SUBSTANCE IN 

 THE SERUM OF IMMUNIZED ANIMALS. 



The serum of animals well immunized against a vibrio contains 

 a preventive substance. This preventive substance is separate 

 from the bactericidal substance also present in the serum. Is the 

 preventive substance likewise specific? In other words will the 

 serum of animals vaccinated against a vibrio other than Koch's, 

 protect against infection with the true cholera organism? The 

 question is still in dispute. Certain observers assert that the serum 

 of animals vaccinated against the vibrio Massaouah protects animals 

 from true cholera; but this is denied by other observers. It is easy 

 enough to perform experiments to elucidate this question, but the 

 interpretations of such experiments are confusing. The studies of 

 Issaeff * have shown us that the injection of certain fluids with no 

 distinct protective power in animals may cause a certain degree of 

 immunity against cholera infection. 



Cholera peritonitis develops rapidly. Guinea-pigs that receive 

 injections of the vibrio are either rapidly killed or else recover 

 rapidly, the struggle between the animal and the parasite being 

 very short. When a sub-lethal dose is given the rapidity of the 

 cure shows that the animal defense is adequate for a time; it has 

 been shown, moreover, that vibrios are quickly transformed within 

 the phagocytes. On the other hand the vibrio multiplies and 

 secretes poisons, and consequently if the animal does not defend 

 * Issaeff, Zeitschrift fur Hygiene, XVI, 1894, 283. 



