168 IMMUNITY. 



According to Metschnikoff, the most important factor in the pro- 

 duction of natural immunity is to be found in the phagocytic action 

 of certain cells within the animal body. He divides phagocytes 

 into mobile and fixed elements. Among the former he places both 

 the mono- and polynuclear leucocytes (with the exception of the 

 small lymphocytes, and the mast cells of Ehrlich), and the so-called 

 wandering cells ; while the fixed phagocytes, or macrophages, consist 

 of endothelial cells, the elements of the spleen pulp and of bone 

 marrow, some connective tissue cells, and possibly certain nerve and 

 muscle cells. He states that sometimes many mononuclear phago- 

 cytes fuse together forming large plasmodia, which may be known 

 as giant cells, and which have a phagocytic action. When bacteria 

 are introduced into a naturally immune animal they are seized upon 

 and devoured by either the mobile or fixed phagocytes, or by both. 

 When a microorganism is introduced into a place where no phago- 

 cytes are present, as for instance under the skin, into the cornea, or 

 into the anterior chamber of the eye, the mobile phagocytes collect 

 at the point of bacterial invasion, engulf the bacterial cells with the 

 aid of their pseudopodia and then digest them or in some other way 

 deprive them of their capability of harming the body. This 

 method of disposing of foreign substances introduced into the ani- 

 mal body is known as phagocytosis. Even those who have brought 

 the best arguments against Metschnikoff 's doctrine of phagocytosis 

 admit that this phenomenon is especially observable in animals 

 which are relatively immune to the invading microorganism, or in 

 other words, they recognize the fact that in natural immunity pha- 

 gocytosis is at least a frequently observed phenomenon. Pfeiffer 

 and KoUe,* in studying the former's reaction, which has already 

 been referred to in our chapter on lysins, state that the greater 

 number of bacteria destroyed in the peritoneum of normal ani- 

 mals are taken up by leucocytes, but that some perish in the 

 exudative fluid. Kruse ^ states that phagocytosis is frequently ob- 

 served, especially in relatively insusceptible animals, and when a 

 weak virus is used. When anthrax bacilli are injected under the 

 skin of a dog a very energetic phagocytosis takes place and indeed, 

 according to Denys and Havet,' this phenomenon may be observed 

 in extra-vascular blood from the dog. Even spores are taken up by 

 the phagocytes and sometimes they develop into bacilli within the 

 phagocyte, thus destroying the organism which has engulfed them, 

 but ordinarily their development is rendered impossible, and their 

 destruction finally accomplished. The phenomenon of phagocytosis 

 can be explained only on the assumption that the phagocyte contains 

 some chemical substance by virtue of which it destroys the captured 



1 Zeitschriftfilr Hygiene, 21, 1896. 

 ^Fliigg^s Mihroorganismen, 1895. 

 'iaCfeWe, 10, 1894. 



