SOME ASPECTS OF THE IMMUNITY QUESTION. 237 



number which shows how many grammes of experimental 

 animal one gramme of protective serum will protect against 

 a certain lethal dose twenty-four hours after the introduction 

 of serum, since injection of this does not immediately yield 

 its full protective value (12). 



Natural immunity undoubtedly is not dependent upon any 

 single cause. It is well known that living vegetable tissues, 

 such as the fruits and tubers of certain plants, most of which 

 have an acid reaction, are free from bacterial growth. 

 Warm-blooded animals are normally refractary to sapro- 

 phytic and putrefactive micro-organisms, which flourish 

 readily on dead tissues, and a natural immunity to the 

 infective pathogenic bacteria of warm-blooded animals is 

 enjoyed by most poikilothermous animals. This natural 

 immunity can, however, be broken down. Frogs and lizards 

 are refractary to anthrax, but as was shown by Gibier 

 {13) and Metschnikoff (14) become susceptible when main- 

 tained at a temperature above 2 5°C, and it has been 

 recently observed that a frog when warmed to 2 5°C. loses 

 its immunity if suddenly changed from a medium of 15 to 

 ■one of 25°, though on gradual transition from 12 to 25 the 

 immune condition is preserved (15). Under certain circum- 

 stances animals naturally immune against definite micro- 

 organisms become invaded by these when the blood 

 corpuscles are destroyed in quantity by such drugs as 

 pyrogallic acid or acetyl-phenyl-hydrazin, and under these 

 •conditions pathogenic germs which locally affect the or- 

 ganism are capable of spreading throughout the whole 

 body (16). The introduction of the blood serum of one 

 animal into the veins of another, the dog into the rabbit, 

 would also produce the same effect. The researches of Leo 

 {17) are well known. He ^attempted to break down the 

 immunity which white mice present to the infection of 

 glanders by treating these with doses of phloridzin, a 

 drug which produces physiological diabetes. This disease, 

 like carcimoma and chlorosis, is accompanied with wasting, 

 though the subjects of the latter diseases are by no means 

 so liable to the invasion of micro-organisms as are dia- 

 betics. Groups of animals were treated with phloridzin 



