LIFE OF ELIE METCHNIKOFF 171 



long and important part of Metchnikoff's scientific 

 achievements. 



The question of immunity is of such great im- 

 portance, the mechanism of this phenomenon and 

 the physiology of intracellular digestion are so com- 

 plicated, that I have thought it useful to epitomise 

 here the exposition given of it by Metchnikoff in his 

 book. Readers who do not care to go further into 

 the subject can pass over the next few pages with- 

 out hindering their comprehension of the following 

 chapters. 



Diseases affect all living beings, and the greater 

 number of plants and animals would cease to exist 

 without innate or acquired immunity. 



Unicellular beings are generally immune against 

 infectious diseases, which are rarely observed in them. 

 Their body being almost entirely made up of digestive 

 protoplasm, the microbes which they absorb are 

 directly introduced into a noxious medium and are 

 destroyed therein like any other food. If the 

 microbes are indigestible, they are immediately 

 rejected ; hence, in the majority of cases, they 

 cannot become harmful. 



This resistance of unicellular beings to many 

 microbes and microbian toxins is due not only to 

 the intense digestive power of the cell but also to 

 the extreme sensitiveness which rules over the choice 

 of food. Owing to this protoplasmic sensitiveness 

 (chimiotaxis) protozoa are attracted towards certain 

 microbes or substances (positive chimiotaxis) and re- 

 pelled by others (negative chimiotaxis). Thus, many 

 ciliate infusoria choose bacteria only for their food ; 

 they are sharply repelled by dead infusoria, etc. 



