246 ACQUIRED IMMUNITY 



in a certain order. Again, if we pass smallpox microbes through 

 a succession of calves, we attenuate the disease. Presumably, we 

 so weaken the toxins that the microbes are unable to spread 

 through the body. Vaccinia, with its weakened toxins confers 

 immunity against smallpox, for, if a man has recovered from the 

 former, his phagocytes are able to destroy the microbes of the 

 latter when brought in contact with them; hence his acquired 

 immunity. In the same way it is possible to immunize against 

 rabies by passing the microbes through a series of animals 

 (monkeys) which 'attenuate 1 it. Again by subjecting anthrax 

 microbes to an abnormal degree of heat we are able to attenuate 

 them. By inoculating first with much attenuated microbes, and 

 then in succession with less attenuated microbes, we are able to 

 bring about immunity against the most virulent form of the 

 malady. Since in these cases, we introduce directly, not anti- 

 toxins but attenuated microbes, then if the theory of chemical 

 neutralization be correct we must make the incredible hypotheses 

 that the passage of rabies through monkeys, of smallpox through 

 calves, and the subjection of anthrax to heat so alters the microbes 

 that they produce substances which exactly neutralize the toxins 

 which their ancestors produced. 



412. It is very significant that microbes, attenuated to the 

 right degree, will confer immunity on animals of a species some- 

 what resistant to the disease, but cause death with symptoms of 

 the virulent disease in animals of a species that is less resistant. 

 A parallel fact is that, while smallpox, measles and many other 

 diseases destroy the less resistant, they confer immunity on the 

 more resistant individuals of the same species. Obviously, there- 

 fore, the toxins of attenuated disease ^still remain toxins but 

 weakened toxins. Consequently we are driven to the conclusion 

 that in rabies, smallpox, and anthrax experience of a mild variety 

 of a disease helps to the acquirement of resisting power against 

 the more virulent variety not by introducing substances which 

 chemically neutralize the toxins but by supplying weakened toxins, 

 reactions against which act as stepping stones towards reaction 

 against strong toxins. 1 In other words, when the individual has 



1 The alteration in the toxin of rabies in drying cords may arise in one of 

 two ways : (i) the virulent toxin may be directly altered by the drying process, 

 or (2) the microbes may be so altered that they produce attenuated toxins. Some 

 authorities have surmized that as drying proceeds the microbes perish, first at 

 the surface of the cord, and then progressively towards the centre, and, therefore, 

 that the efficacy of Pasteur's treatment depends on the injection of small but 

 progressively increasing .^quantities of microbes. But the microbes are already 



