246 SUSCEPTIBILITY AND IMMUNITY. 



by any means find their way into the blood. The propensity exhib- 

 ited by the leucocytes for picking up inorganic granules is well 

 known, and that they may be able not only to pick up but to assimi- 

 late, and so dispose of, the bacteria which come in their way, does 

 not seern to me very improbable, in view of the fact that amcabsB, 

 which resemble them so closely, feed upon bacteria and similar or- 

 ganisms." ' 



At a later date (1884) Metschnikoff offered experimental evi- 

 dence in favor of this view, and the explanation suggested in the 

 above quotation is commonly spoken of as the Metschnikoff theory. 



The observations which first led Metschnikoff to adopt this view 

 were made upon a species of daphnia which is subject to fatal infec- 

 tion by a torula resembling the yeast fungus. Entering with the 

 food, this fungus penetrates the walls of the intestine and invades the 

 tissues. In certain cases the infection does not prove fatal, owing, as 

 Metschnikoff asserts, to the fact that the fungus cells are seized upon 

 by the leucocytes, which appear to accumulate around the invading 

 parasite (chemiotaxis) for this special purpose. If they are success- 

 ful in overpowering and destroying the parasite the animal recovers ; 

 if not, it succumbs to the general infection which results. In a simi- 

 lar manner, Metschnikoff supposes, pathogenic bacteria are destroyed 

 when introduced into the body of an immune animal. The colorless 

 blood corpuscles, which he designates phagocytes, accumulate at the 

 point of invasion and pick up the living bacteria, as they are known 

 to pick up inorganic particles injected into the circulation. So far 

 there can be no doubt that Metschnikoff is right. The presence of 

 bacteria in the leucocytes in considerable numbers, both at the point 

 of inoculation and in the general circulation, has been repeatedly 

 demonstrated in animals inoculated with various pathogenic bacteria. 

 The writer observed this in his experiments, made in 1881, in which 

 rabbits were inoculated with cultures of his Micrococcus Pasteuri ; 

 and it was this observation which led him to suggest the theory 

 which has since been so vigorously supported by Metschnikoff. But 

 the presence of a certain number of bacteria within the leucocytes 

 does not prove the destructive power of these cells for living patho- 

 genic organisms. As urged by Weigert, Baumgarten, and others, 

 it may be that the bacteria were already dead when they were picked 

 up, having been destroyed by some agency outside of the blood cells. 

 As heretofore stated, we have now experimental evidence that blood 



1 " A Contribution to the Study of Bacterial Organisms commonly found upon 

 Exposed Mucous Surf aces and in the Alimentary Canal of Healthy Individuals." Il- 

 lustrated by photomicrographs. Proceedings of the American Association for Ad- 

 vancement of Science, 1881, Salem, 1882, xxx., 83-94. Also in Studies from the 

 Biological Laboratory, Johns Hopkins University, Baltimore, vol. ii.. No. 2, 1882. 



