630 -THE POPULAR SCIENCE MONTHLY. 



nic 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 Theoky of Phagocytosis. The fact that in certain in- 

 fectious diseases due to bacteria the parasitic invaders, at the point 

 of inoculation or in the general blood-current, are picked up by 

 the leucocytes, and in properly stained preparations may be seen 

 in their interior, has been known for some years. 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 a hypothesis as long ago as 1881, 

 in a paper read August 18, 1881, before the American Association 

 for the Advancement 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 

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

 hibited by the leucocytes for picking up inorganic granules is 

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

 assimilate, and so dispose of, the bacteria which come in their 

 way, does not seem to me very improbable, in view of the fact 

 that amoebae, which resemble them so closely, feed upon bacteria 

 and similar organisms." 



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 in- 

 fection 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 accumu- 

 late around the invading parasite (chemiotaxis) for this special 

 purpose. If they are successful in overpowering and destroying 

 the parasite, the animal recovers ; if not, it succumbs to the gen- 

 eral infection which results. In a similar 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 inva- 

 sion 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 bac- 

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

 of inoculation and in the general circulation, has been repeatedly 



