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 
