CAUSES OF DISEASE. 29 



cells in the blood take in both the prevention and cure 

 of infections. These cells (leucocytes), which he 

 called phagocytes, he believed acted through their 

 power of taking up and digesting bacteria. During 

 all these years since the publication of his theory of 

 phagocytosis Metchnikoff has added many valuable 

 facts corroborating his views. In the light of the 

 recently discovered antibody, opsonin, his views on 

 the protective function of the leucocytes are strongly 

 supported, in that data for the first time is supplied 

 which explains the conditions under which phagocy- 

 tosis is active. Opsonin in the blood-serum of immune 

 animals and persons sensitizes or opsonizes the injec- 

 tions agents for easier destruction by the phagocytes. 



Injuries to tissues per se produce symptoms, through 

 the disturbance of functions which such loss to the 

 economy entails; but besides these symptoms, there 

 are others, in every infectious disease, which can be 

 definitely ascribed to the production of the defensive 

 substances. The phenomena of disease, therefore, 

 are of a composite character, comprising both the 

 symptoms of injury and those of defense. What part 

 of the symptoms each of these factors causes in a case, 

 it is impossible to determine, but of this we may always 

 feel sure (because the experimental proofs in its favor 

 are conclusive) no insignificant part of the fever and 

 general malaise in every infectious disease is a necessary 

 adjunct to antibody production. This fact lends the 



