IMMUNITY AND SUSCEPTIBILITY 115 



attack by other diseases — thus, diphtheria and scarlet fever 

 mutually predispose to one another. 



Hypotheses of Immunity. — Before examining in detail the 

 chief theories that have been put forward to account for 

 the phenomenon of immunity, we may first consider certain 

 special cases of immunity which merit individual attention. 



It has been found experimentally that frogs, which are 

 naturally immune to anthrax, may be infected if they are 

 kept at a temperature of 37" C. ; fowls, on the other hand, 

 may be made susceptible to the disease by being immersed 

 in water so as to lower their temperature. This would 

 appear to point to the presence of some means of resistance 

 normally present ia frogs and fowls which ceases to be 

 effective when their normal conditions of existence are 

 interfered with. 



It has been found by several observers that the blood of 

 various animals possesses decided germicidal powers ; for 

 example, the blood of rats is able to destroy the vitality of 

 anthrax bacilli, though this property is not impossibly due 

 to its excessive alkalinity. 



The blood of animals immune to anthrax has been found 

 to have the action of conferring protection against a dose 

 of anthrax bacilli that would otherwise be fatal if injected 

 with it, or within a certain interval of time before or after 

 injections. Blood serum from animals immune to a disease 

 may, however, have the effect of conferring a power of 

 resisting the action of the toxalbumens produced by that 

 organism without having any germicidal action on living 

 organisms themselves ; thus, diphtheria antitoxin serum 

 injected into a suitable animal will neutralize the effect 

 of the poisonous toxalbumen in diphtheria cultures, but it 

 has not a germicidal effect on the bacilli, as diphtheria 

 antitoxic serum may be used as a culture medium for the 

 Klebs-Loffler bacillus. 



