MECHANISM OF ACTIVE IMMUNIZATION 661 



fection the host endeavors to protect itself by producing antibodies. 

 When the protection is insufficient, the infection will spread; when the 

 antibodies are in excess, the infection is overcome; when the forces are 

 about equal, a stage of chronicity may result in which the host becomes 

 accustomed, as it were, to the invaders, and, while the infection does 

 not spread rapidly, it does not, on the other hand, recede. In cases of 

 the latter type an extra dose of bacterial stimulant (a bacterin) may 

 arouse dormant or inactive cells to furnish an extra quantity of anti- 

 bodies and thus turn the tide. In acute infections indifferent groups of 

 cells may likewise be brought into action, although it is more likely that 

 they are already involved, so that the extra stimulation in the form of 

 bacterin must be cautiously and carefully applied, if applied at all. 



In therapeutic inoculation, therefore, the fundamental principle is to 

 stimulate in the interest of the infected tissues the unexercised immunizing 

 capacities of the uninfected tissues. This is especially true hi chronic 

 infections, when the use of a bacterial vaccine may be likened to the 

 application of the whip to a lazy horse that is capable of further effort 

 and work. In acute infections, however, while the cells are at work 

 they may be capable of greater effort, but vaccines should be given 

 cautiously, as they may, to use the same simile, act as a whip to a willing 

 and well-worked horse that is unable to respond or does respond, with 

 resulting disastrous overexertion. 



It should be remembered, in this connection, that usual forms of treat- 

 ment should be given while bacterin therapy is being instituted. For in- 

 stance, it is useless to administer a vaccine to a patient with a sup- 

 purative fistula or sinus if an infected silk suture is directly responsible 

 for the suppuration. The suture should be removed, if possible, and 

 after this is done, a vaccine may be of considerable aid in overcoming 

 the coincident infection. 



In what manner can dead bacteria cause the production of anti- 

 bodies? The mechanism is similar to that involved during infection 

 with the living microorganism, and involves the first principles of im- 

 munity. The antigenic powers of a vaccine are probably always more 

 or less inferior to the living antigen, as some principle may be lost during 

 heating, drying, passage through animals, the action of germicides, etc. 

 For this reason living vaccines are to be preferred, although, for obvious 

 reasons, they cannot generally be employed in human practice. 



Just what portion of the bacterial cells is mainly antigenic it is 

 difficult to determine, for it probably varies with different species. 

 With a true toxin, as, for example, the diphtheria bacillus, the toxin 



