i8 Predisposition and Immunity. 



sues of the immune animal have no affinity^ or but little affinity 

 (chemical affinity), toivard the toxines of the infection. The 

 poisonous elements simply do not enter into combination with 

 them. For example, the nervous system of the turtle is absolutely 

 immune to the toxines of diphtheria and tetanus, and these sub- 

 stances may be injected into the animal entirely without effect. 

 Yet the toxines thus introduced may remain in the bodies of the 

 experiment animals for months without being rendered inert by 

 the juices ; and should the blood of such a turtle be injected into 

 some susceptible animal it will act in the sarne manner upon the 

 latter as would the toxine itself, 



FYom a second point of view it is to be recognized that im- 

 munity may depend upon the fact that some of the cells of the 

 animal in question are able to take up and digest micro-organisms, 

 and thus render them harmless (Phagocytosis). This power of 

 seizing and ingesting small particles, organic and inorganic, or 

 dead and living cells, is peculiar especially to the motile types of 

 leucocytes (wandering cells, white blood cells), but is also 

 possessed by giant cells, splenic and medullary cells, and even 

 fixed connective tissue cells (as endothelium) ; and plays an 

 important part in the economy of the body. Its significance has 

 been pointefl out especially by the ingenious investigations of 

 Metschnikoff, Leber, and Bordet. These phagocytes act as scaven- 

 gers, taking up and making awa}' sometimes with blood debris, 

 nuclear fragments, pigment, fat globules and all sorts of minute 

 foreign particles with which they come in contact. The ingestion 

 of such corpuscular elements is with them a simple process of 

 feeding. The movement and approach of the phagocytes may 

 be induced bv a number of stimuli, as warmth or an acid, acting 

 upon their own tactile or chemical sensitiveness. Tn coming in 

 contact with foreign particles they attempt to increase their 

 surface of contact as much as possible [applying their protoplasm 

 more and more about the surface of the particle, and thus 

 eventually enveloping it in their own material] ; and are at- 

 tracted by various chemical substances (cheniota.vis) . [It is 

 but fair to add here that while the theory of chemotaxis, as a 

 part of the general theory of "tropisms" or of blind automaton- 

 like response of living things to external forces or attractions, 

 finds wide adherence among medical men and biologists, there 

 are nevertheless others who do not accept such a view, the latter 

 finding reason to believe that the manifestations which the former 



