SUSCEPTIBILITY AND IMMUNITY. 245 



genie bacteria. But, as already stated, in the light of recent experi- 

 ments this theory now appears to us to be untenable as a general 

 explanation of acquired immunity. 



The Theory of Phagocytosis. The fact that in certain infectious 

 diseases due to bacteria the parasitic invaders, at the point of inocu- 

 lation or in the general blood current, are picked up by the leuco- 

 cytes and in properly stained preparations may be seen in their in- 

 terior, has been known for some years. In mouse septicaemia an 

 infectious disease described by Koch in his work on "Traumatic 

 Infectious Diseases," published in 1878 the slender bacilli which are 

 the cause of the disease are found in large numbers in the interior of 

 the leucocytes. Koch says, in the work referred to : " Their rela- 

 tion to the white blood corpuscles is peculiar ; they penetrate these 

 and multiply in their interior. One often finds that there is 

 hardly a single white corpuscle in the interior of which bacilli can- 

 not be seen. Many corpuscles contain isolated bacilli only ; others 



FIG. 78. Bacillus of mouse septicaemia in leucocytes from blood of mouse (Koch). 



have thick masses in their interior, the nucleus being still recog- 

 nizable ; while in others the nucleus can be no longer distinguished ; 

 and, finally, the corpuscle may become a cluster of bacilli, breaking 

 up at the margin the origin of which one could not have explained 

 had there been no opportunity of seeing all the intermediate steps 

 between the intact white corpuscle and these masses " (Fig. 78). It 

 will be noted that in the above quotation Koch affirms that the 

 bacilli penetrate the leucocytes and multiply in their interior. Now, 

 the theory of phagocytosis assumes that the bacilli are picked up by 

 the leucocytes and destroyed in their interior, and that immunity de- 

 pends largely upon the power of these " phagocytes" to capture and 

 destroy living pathogenic bacilli. 



The writer suggested this as an hypothesis as long ago as 1881, 

 in a paper read before the American Association for the Advance- 

 ment of Science, in the following language: 



" It has occurred to me that possibly the white corpuscles may 

 have the office of picking up and digesting bacterial organisms which 



