ACQUIRED IMMUNITY 173 



Passive immunity is of shorter duration than active immunity, and 

 the former is especially indicated in prophylaxis for warding off an 

 acute infection that has a relatively short incubation period. The de- 

 gree of passive immunity is also seldom equal to that of an active im- 

 munity. The antibodies produced by our own cells are more lasting 

 and possess higher protective value. This is an important factor in 

 von Behring's method of immunization in diphtheria, when a small 

 amount of toxin loosely bound to antitoxin is injected in the belief that 

 the toxin becomes dissociated and serves to stimulate our body-cells into 

 producing our own antitoxin. 



Passive acquired immunity is usually antitoxic, as, for example, 

 that induced by the administration to -man of diphtheria antitoxin 

 prepared by the body-cells of the horse. Antibacterial serums may 

 likewise induce a passive immunity, as, for instance, that used in im- 

 munization against plague. 



It is evident, therefore, that the processes whereby infections are 

 overcome and immunity is conferred, and the general reactions that 

 follow the introduction into the body of modified antigens in the prac- 

 tice of immunization, are complex processes, and in none is one anti- 

 body produced or solely responsible for the resulting immunity. The 

 properties and action of the known antibodies are considered in sub- 

 sequent chapters, particular attention being given to methods for de- 

 termining their presence in the body-fluids, which serve as an aid to the 

 diagnosis of infection as based upon the general law that the antibody 

 is specific for its antigen, and so, when the presence of an antibody 

 is demonstrated, it may be assumed that the antigen is or has been 

 present. 



Nothing is known concerning the nature of the immunity that is ac- 

 quired against several infections, such as scarlet fever, measles, small- 

 pox, etc., nor will much be known until the causes that give rise to these 

 conditions have been discovered. 



Theory of Vaughan. According to Vaughan, the inability of a bac- 

 terial cell to grow in the animal body either because it cannot feed upon 

 the protein of the body or because it is itself destroyed by the ferments 

 elaborated by the body-cells explains all forms of bacterial immunity, 

 either natural or acquired. Thus in antitoxin immunity the toxin is 

 regarded as a ferment that splits up the proteins of the body-cells, setting 

 the protein poison free. The body-cells react with the formation of an 

 antiferment or antitoxin, which neutralizes the toxin and prevents 

 cleavage. The toxin itself is regarded as harmful only in so far as it is 



