94 PATHOGENIC BACTERIA. 



both of which diseases are highly infectious for the 

 Caucasian. 



Explanation of Natural Immunity. — There are two 

 agencies upon either or both of which natural immunity 

 may depend ; first, the cells ; second, the humors of the 

 body. 



I. The Activity of the Cells. — Phagocytosis. — The 

 ameboid movements of the leukocytes and their ability to 

 take inert particles into their protoplasm were observed 

 by Virchow a half century ago while he was working 

 upon the details of the cellular pathology. Carl Roser as 

 early as 1881 observed that the leukocytes sometimes take 

 up bacteria ; and a little later Sternberg, Koch, and others 

 corroberated his observations. That this evidence of a 

 phagocytic action by the cells might have any bearing 

 upon immunity seems to have first occurred to the biolo- 

 gist Metschnikoff, who, viewing the phenomena with a 

 broad biological horizon, recognized in the activities of 

 the leukocytes and related cells of the higher animals, 

 processes universal among the unicellular animal organ- 

 isms. From comparative studies of cellular processes in 

 high and low forms of life, Metschnikoff concluded that 

 their importance in preserving the health of the organ- 

 ism by destroying the cause of disease could not be over- 

 estimated, and elaborated the theory of phagocytosis, now 

 inseparably associated with his name. Certain of his 

 observations are familiar to every one who has studied 

 biology and observed the ameba with its incorporated 

 diatomes, desmids, etc., and the food vacuoles in which 

 the digested products of similar organisms are contained; 

 or observed the myriads of bacteria and other minute 

 organisms flowing into the mouth opening of the Parame- 

 cium, rotifier, vorticella, etc. All of these unicellular 

 animals incorporate, destroy, and digest bacteria. Why, 

 therefore, may not the phagocytic cells of the higher 

 animals be endowed with similar powers? 



Indeed, the analogy is complete. The leukocytes — -es- 

 pecially the polymorphonuclear and eosinophilic forms, to 



