158 INFECTION AND IMMUNITY. 



bit's liver is able to fix a great deal of the toxin. 

 And Metchnikoff lias found that the liver of the 

 scorpion, which has an absolute immunity to tet- 

 anus, absorbs the toxin and retains it for months. 

 summary. We may, then, enumerate the following as the 

 factors which probably are responsible for the dif- 

 ferent grades of natural immunity and suscepti- 

 bility to various bacteria and their toxins: the 

 bactericidal and antitoxic powers of the serum and 

 plasma; the destructive effects of the phagocytes 

 and other cells on both bacteria and toxins; a pos- 

 sible absolute non-susceptibility in some cases (the 

 absolute non-existence of suitable cell receptors) ; 

 the lack of suitable available food for the micro- 

 organisms in some instances (atreptic immunity: 

 see the preceding chapter) ; the overwhelming dis- 

 tribution of the "suitable" cell receptors in organs 

 of less vital necessity for the individual, thus 

 diverting the poisons from the more important 

 organs. 



This knowledge is very general, however, and in 

 many specific instances we continue to be in doubt 

 regarding the exact conditions which are respon- 

 sible for natural immunity and susceptibility. We 

 have no reason to believe that any one factor is 

 operative for all infections, although phagocytosis 

 appears to be more general in its action than the 

 other processes mentioned. Each disease must be 

 studied as a unit in relation to each species of 

 animal. In one instance the resistance or suscepti- 

 bility may depend on the bactericidal power of the 

 body fluids; in another, on the germicidal action 

 of the leucocytes and other cells ; in another, on the 

 antitoxic (destructive) action of the cells or fer- 

 ments with or without the presence of true "anti- 



