IMMUNITY AND ANAPHYLAXIS 185 



the protective substances from an immunized animal, 

 because, in the first case, an immunizing apparatus has 

 been set up which is able to produce an (apparently) 

 indefinite amount of protective substances and over a 

 long period, whereas in the second case, a definite amount 

 of the protective substances is injected, and when this 

 amount is used up the protection is at an end. In active 

 immunization, we must distinguish a " specific immunity 

 to bacteria," and a " specific immunity to their toxins," 

 just as we have a natural resistance or immunity to 

 bacteria, which is different from the natural resistance or 

 immunity to certain poisons. Thus the immunity after 

 diphtheria is mainly to the toxins (antitoxic). On the 

 other hand, the immunity purchased by the injection of 

 cholera vibrios, is merely to the bacteria and not to their 

 endotoxins. Hence the injection of cholera spirilla into an 

 animal previously immunized to the same, is followed by 

 the death and dissolution of the spirilla, but if the dose is 

 large enough, also by a fatal intoxication of the animal 

 by the cell poisons thus suddenly set free. This is the chief 

 cause of failure of immunization to those bacteria which 

 do not act (as diphtheria and tetanus do) through soluble 

 toxins, diffused into the blood stream. The immunization 

 would require, in such cases, to be of a double nature, 

 namely, antibacterial and antitoxic. Apparently, as 

 usually induced, the former mainly, if not entirely, results. 

 The methods of active immunization are based on the 

 work and discoveries of Pasteur and his associates, and 

 may be summarized thus : 



(i) With living bacteria, virulent or attenuated. 



(2) With dead bacteria. 



(3) With the bacterial cell substances. 



(4) With soluble toxins or filtrates. 



(5) By feeding with toxic substances. 



1. With Living Bacteria or Virus, Virulent or Attenuated. 

 (a). With virulent virus. Though the virus of small-pox 

 is still unknown with certainty, inoculation of the small- 

 pox, as introduced into England in 171 8 by Lady Mary 

 Wortley Montagu (see her " Letters "), may be given as an 

 example. The inoculated disease was usually mild in type, 



