248 ACQUIRED IMMUNITY 



Presumably the same digestive substance which destroys them 

 when enclosed within the phagocytes is secreted into the surround- 

 ing medium and destroys them there just as pepsin, which is 

 harmless to the stomach cells which produce it, digests food at a 

 distance from them. Moreover, just as pepsin digests both snake 

 venom and the gland which secretes the venom, so, presumably, 

 the ferments secreted by the phagocytes digest both the microbes 

 and their toxins. Of the existence* of these digestive ferments 

 there can be no doubt. Thus, if we add blood from a person 

 recovering from enteric fever to a medium in which typhoid bacilli 

 are contained, the behaviour of the latter indicates injury. 



414. If, then, this theory be correct, and I am not aware of 

 anything known to us which contradicts it, antitoxic serum from 

 a horse contains substances ferments and enzymes which are 

 engaged in digesting the toxins and so attenuating them ; for 

 which reason a fatal dose of toxin is rendered harmless when 

 mixed before injection with its equivalent of antitoxin while the 

 effect of even larger doses is mitigated. But, what is even more 

 important, it contains more or less attenuated toxins which form a 

 scale up which the cells of the individual into whom it is injected 

 react until he achieves the power of tolerating the strongest toxins. 



415. Given suitably attenuated toxins and time for reaction, 

 immunity against acute disease may almost always be secured. 

 Thus, if an individual be infected with rabies, which develops 

 slowly, there is usually ample time if treatment be begun early. 

 In diphtheria, which develops quickly, days and hours may be 

 of vital importance ; but since we are able to pour in an ample 

 supply of antitoxin, our treatment may be very successful. The 

 vaccine treatment of smallpox, on the other hand, is preventive 

 not curative. It is useless to vaccinate after the disease has 

 manifested itself, for before the weaker toxin is elaborated, the 

 sufferer is on the high road to recovery, or is past help. 1 Pre- 

 sumably the normal process of acquiring immunity is in essence 

 similar to that which occurs when antitoxins are artificially intro- 

 duced. The severity of the disease depends in part on the resisting 

 power of the individual attacked, and in part on the virulence of 



1 It is remarkable that subcutaneous injections of considerable quantities of 

 vaccine lymph does not appear to have been tried in smallpox. Theoretically 

 such treatment should be excellent. It is probable, however, that the majority 

 of deaths amongst sufferers from smallpox are caused not by that disease, but by 

 subsequent septic infection and the absorption of the toxins of putrefaction. 

 Even so, however, if the injection of vaccine lymph caused the vesicles to abort, 

 the effect wouM be curative. 



