208 A MANUAL OF BACTERIOLOGY 



and bacteriolysis, are better marked than with most other 

 bacteria, and polynuclear phagocytosis seems subsidiary. 



Antitoxin formation probably plays little or no part 

 in acquired immunity, or even in recovery from infection. 

 In diphtheria, for instance, antitoxin is not found until 

 the disease has subsided. Possibly, in chronic infections, 

 antitoxin formation does play a subsidiary role in recovery. 



To sum up, natural immunity is probably due to a 

 number of factors, some or all of which may be operative 

 in particular instances, and it is impossible to state with 

 certainty any general law. In most cases phagocytosis 

 is the principal means of defence, the germicidal, inhibi- 

 tory, or bacteriolytic actions of the body-fluids aiding, 

 though of subsidiary importance ; in others the cells and 

 tissues are unaffected by the bacterial toxins, sometimes 

 because the cells are lacking in the particular side- chains 

 or receptors which fix the toxin ; sometimes because, for 

 some unknown reason, the cells are unaffected by the 

 toxophore group of the toxin. 



As regards the immunity acquired after an attack of 

 disease, this may be due to the " education " of the leuco- 

 cytes, whereby they are attracted, whereas formerly 

 repelled, by the products of bacterial development, or to 

 substances which stimulate the action of the leucocytes. 

 The germicidal, inhibitory, and bacteriolytic actions of the 

 body-fluids may also be enhanced. It seems probable 

 also in certain instances that the side- chains or receptors 

 having an affinity for the toxin become in some way 

 destroyed or used up, so that further fixation of the 

 particular toxin cannot take place. 



It is to be noted, as Metchnikofl: has pointed out, that 

 immunity is much more rapidly acquired against micro- 

 organisms than against their toxins. In Nature, it is 

 principally against micro-organisms that the body requires 

 protection. 



