240 ILLINOIS ACADEMY OF SCIENCE 



corpuscles more and more digested in the large mononuclear 

 cells of the guinea pig, demonstrating that certain cells of the 

 Mammalian animals possess phagocytic properties. 



Metchnikoff's opponents tried to disprove this by showing 

 that in animals dead from anthrax, numerous anthrax bacilli 

 were seen in the blood, none of which were being or had been 

 taken up or digested by phagocytic cells. Metchnikoff then 

 succeeded in demonstrating that in animals not susceptible 

 to anthrax such phagocytosis of bacilli took place, and that 

 the lack of susceptibility depended upon the fact the anthrax 

 bacilli were taken up and destroyed by the phagocytes. Upon 

 this fact the use of vaccines in medicine is based, the vaccine, 

 being the substance used to increase the phagocytic powers of 

 the body. 



Non-susceptibility to a given disease or a given organism or 

 toxin, either under natural conditions or under conditions ex- 

 perimentally produced, is called immunity. Immunity is, in 

 fact, of widely varying degrees and has correspondingly rela- 

 tive significance. So long as an organism continues to exist, 

 it must continue to adapt itself to its invironments, and thus 

 it becomes so modified as to effectually resist influences which, 

 without such modification, would have brought cessation of 

 being. The lower animals are immune to some diseases pre- 

 valent in man and certain families have marked resistance to 

 some diseases. These are examples of natural immunity. An 

 individual may be immune by virtue of his being of a certain 

 race or family. Certain animals may possess a congenital 

 natural immunity. For instance, many warm blooded animals, 

 such as guinea pigs, cattle, mice, and rabbits, are susceptible 

 to anthrax, while dogs and rats possess quite a strong, though 

 not an absolute natural immunity against this infection. Man 

 is susceptible to typhoid bacillus and cholera spirillum infec- 

 tions, while all our domestic animals are immune against these 

 so far as natural infection is concerned. Classic is the obser- 

 vation, on the other hand, that one attack of a certain infec- 

 tious disease affords lifelong immunity against attack of the 

 same disease, while in other diseases the acquired immunity 

 is varying in duration. Persons who have had one attack of 

 measles, scarlatina, typhoid fever, and smallpox are generally 



