2$0 BACTERIA 



or a voluntary acquired immunity. For example, the 

 former is at once illustrated by an attack of the disease. 



Small-pox, typhoid fever, even scarlet fever, are diseases 

 which very rarely attack the same individual twice. That 

 is because each of these diseases leaves behind it, on its first 

 appearance, its antitoxic influence. Hence the individual 

 has involuntarily acquired immunity against these diseases. 

 An example of voluntary acquired immunity is also at hand 

 in the old method of preventive inoculation for small-pox, 

 or variolation. This was clearly an inoculation setting up 

 an artificial and mild attack of small-pox, by which the 

 antitoxins of that disease were produced, and protected the 

 individual against further infection of small-pox ; that is to 

 say, it was a voluntary acquired immunity. This form of 

 artificial production of protection is generally called art- 

 ificial immunity. Let us now marshal together these various 

 terms in a table as follows : 



j- -, \ = a condition of protection of insusceptibility to cer- 



1. Natural immunity = constitutional protection produced by alexines. 



2. Acquired immunity 



Acquired naturally (involuntarily) produced by antitoxins formed by 



an attack of the disease. 

 Acquired artificially (voluntary)= 



(a) Active immunity, produced by direct inoculation of the weak- 



ened bacteria or weakened toxins of the disease, e. g., vac- 

 cination, or Pasteur's treatment of rabies, or Haffkine's 

 inoculation for cholera. 



(b) Passive immunity, produced by inoculation, not of the disease 



or of its toxins, but of the antitoxins produced in the body 

 of an animal suffering from the specific disease. 



It is hoped that previous remarks will have explained the 

 meaning of the terms used in the above table, with the 

 exception of the last two phrases of active and passive im- 

 munity. We propose now to consider in some detail the 

 four illustrations quoted under these two headings, viz., 



