GENERAL PHENOMENA OF IMMUNITY 25 



endotoxins are probably fallacious, because of the mixture of un- 

 recognized exotoxins, the latter producing the immune reaction. 



Active immunization may be produced not only by toxic sub- 

 stances elaborated by bacteria, but also by toxic substances produced 

 in animal life, such as snake venoms, spider poisons, and similar 

 substances. Higher plant poisons, such as ricin, abrin, crotin, etc., 

 may produce specific neutralizing antibodies. The practical value 

 of the antitoxins prepared against bacterial toxins and against the 

 venoms produced by animals is such as to have added greatly to the 

 combating of poisoning by these substances. 



Passive Immunization. In active immunization the animal 

 manufactures within its own body immune substances which serve 

 to protect against and combat infection. Passive immunization, 

 however, utilizes these immune substances, through the transfer of 

 blood serum containing the products of active immunization. The 

 most common example of passive immunization is found in the 

 therapeutic use of diphtheria antitoxin. For practical purposes the 

 diphtheria antitoxin is manufactured in the body of the horse. The 

 injection of immune horse serum transfers to man the immunity 

 actively produced in the horse. Passive immunity of this sort serves 

 to protect against infection, and until the possibility of active im- 

 munization of man against diphtheria was demonstrated, the former 

 method was widely employed for protection of exposed individuals 

 against diphtheria. This method of protection has the great advan- 

 tage of quickly conferring immunity and is widely employed when 

 time does not permit the use of methods for developing active im- 

 munity. After the disease has developed the use of immune serum 

 to combat the infection has the utmost value. In the case of tetanus 

 antitoxin the protective value of prophylactic injections has been 

 amply demonstrated, but in this instance the great affinity between 

 nerve tissues and tetanus antitoxin is such that the therapeutic use 

 of tetanus antitoxin after the disease has developed has not given 

 such beautiful results as has been true of the serum treatment of 

 diphtheria. Much encouragement has recently been afforded by the 

 use of similarly prepared antitoxins against the toxin of the bacillus 

 of gas-gangrene, and there is little doubt that the methods may be 

 much more widely employed as it becomes possible to demonstrate 

 the formation of true exotoxins by other bacteria. Not only may 

 advantage be taken of substances produced by artifically acquired 

 immunity, but in certain instances it is feasible to use the blood 

 serum of individuals who have acquired immunity by survival of an 

 attack of certain diseases. In this field, however, the facts have not 

 been accumulated in sufficient number to justify unqualified ap- 

 proval of the method. 



Passive immunity may be not only antitoxic in character, but 

 also anti-bacterial. Anti-bacterial immune sera have been prepared 

 against the streptococcus, the meningococcus, the pneumococcus, 

 and other organisms. The success with passive immunization by 



