26 



STUDIES IN IMMUNITY. 



when it bears some definite relation to a condition of resistance. It 

 often happens, as we know, that the serum of an animal is bacteri- 

 cidal for a given micro-organism, although the animal itself is quite 

 susceptible to infection by that very organism. For example, 

 rabbit serum is bactericidal for the anthrax bacillus, although the 

 rabbit is highly susceptible to this bacterium.* 



This is simply an example of the spontaneous bactericidal power 

 found in normal animals; its significance from the standpoint of 

 immunity is not clearly established, and its importance is doubtless 

 exaggerated. 



We shall consider then, more particularly the strong bacteri- 

 cidal property in animals immunized against certain infections, 

 for example, against cholera. The bactericidal property of the 

 serum of normal animals against the cholera vibrio is insignificant. 

 Following vaccination the bactericidal property becomes very 

 distinct. Since it is very evident during the stage when the 

 animal is insusceptible to infection, its relation to immunity is 

 unmistakable. 



* The aqueous humor of the rabbit is bactericidal for anthrax. This fluid 

 contains few or no leucocytes; and it would seem, therefore, difficult to attribute 

 the bactericidal property to cells. But it is to be noted that bacteriolysis occurs 

 in this instance in a highly susceptible animal, and obviously, therefore, bears no 

 relation to a condition of resistance. It differs, also essentially, from the bacte- 

 riolysis that occurs in certain instances of immunity, as the property in this in- 

 stance resists heating, to 60 degrees for an hour. The bactericidal property would 

 seem to be more like that shown by such liquids as bouillon. 



It is evident from the following table that the aqueous humor of the rabbit 

 even after heating for an hour to 60 C. is quite bactericidal for B. anthracis. 



EXPERIMENT 4. Aqueous humor was taken from a rabbit. Part of it was 

 heated to 60 degrees for an hour, and the rest was not heated. Equal amounts 

 of each fluid were placed in tubes. A culture of non-spore-forming anthrax 

 bacilli, 24 hours old, was suspended in 0.6 per cent NaCl solution and planted in 

 these tubes. Gelatin plates were made at intervals. 



NUMBER OF COLONIES. 



