ACTIVE AND PASSIVE IMMUNITY 241 



been made. (/) An injection of a right quantity of antitoxin 

 confers a ' passive ' immunity which is less enduring than the 

 'active' immunity which results from actual experience of the 

 disease. Some authorities suppose that the former is due merely 

 to the presence of neutralizing substances introduced with the 

 injection of antitoxin, or afterwards produced under its influence 

 by the individual, and that it persists only so long as these remain 

 within the system ; whereas active immunity is thought to be a 

 vital reaction to the microbes themselves. But probably the truth 

 is, that active and passive immunity are merely degrees of the 

 same thing, and that in any disease the duration of immunity 

 depends more on the severity of the illness, and on the consequent 

 completeness of the reaction, than on the presence or absence of 

 the microbes. A kind of passive immunity may be induced by 

 the repeated injection of small quantities of microbes. Workmen 

 in sewers acquire a passive immunity to enteric fever, doubtless 

 through constant experience of small doses of bacilli. Dwellers in 

 malarious countries, or in places where yellow fever is prevalent, 

 live immune till after a sojourn in a more healthy climate. One 

 vaccination mark confers an immunity which is more passive than 

 that conferred by two, that conferred by two is less enduring than 

 that conferred by more, and still less lasting than that acquired 

 through experience of actual smallpox. Judged by its duration, 

 immunity against such diseases as diphtheria, influenza, and 

 common cold is always passive. 



404. (m) Just as immunity acquired against any disease does not 

 confer immunity against any other, just as vaccinia confers im- 

 munity only against smallpox, so the antitoxin of any one disease 

 confers no resisting power against any other, (n) Within limits, a 

 dose of toxin large enough to be poisonous may be rendered 

 harmless by previous intermixture with the corresponding anti- 

 toxin. This ' neutralization ' is somewhat less marked if the two 

 be simultaneously injected into separate parts of the body, and 

 still less marked if the antitoxin be injected some time after the 

 toxin which, in effect, is what is done when disease is treated by 

 means of appropriate serum. A mixture of toxin and antitoxin, 

 for example that of tetanus, in a proportion just harmless to a 

 rather resistant animal of a susceptible species, for example a 

 mouse, is poisonous to a more susceptible animal of the same 

 species, or to an animal of a more susceptible species, for example 

 a guinea pig. An animal which has acquired immunity to a 

 disease (i.e. which has acquired the power of resisting infection) 

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