620 ACTIVE IMMUNIZATION 



and aids in destroying the protein of the infecting microorganism and its 

 tissues. 



Living versus Dead Vaccines. Barring accidents, the employment 

 of a living virus is the most certain way of calling forth a maximum 

 output of antibodies. There is at present no satisfactory explanation 

 for this except that heat-labile substances destroyed in the ordinary 

 preparation of bacterial vaccines have antigenic properties (Smith). 

 Living vaccines are also capable of penetrating into deeper tissues, 

 whereas dead vaccines may remain where they are deposited. Similarly 

 living viruses are capable of exerting a continuous action and of de- 

 livering an infinite number of blows, whereas the injection of a dead 

 virus produces an interrupted action and deals but a single blow. The 

 actual dangers of using a living vaccine, as the possibility of it being 

 too virulent and thus producing disease, or of regaining virulence or 

 producing chronic " carriers" preclude their general employment in 

 human practice. 



Sensitized Vaccines. Besredka and Metchnikoff 1 have suggested 

 a plan of injecting a vaccine composed of living bacteria that have been 

 immersed in their specific immune serum or, in other words, have been 

 sensitized (serobacterin) . They believe that such vaccines produce 

 practically no negative phase, but only slight local and general reaction, 

 and that the general response with antibody formation is facilitated. 

 This principle is supported by the observations of Theobald Smith, 

 who found that the experimental injection of a toxin-antitoxin mixture 

 aids the dissemination of the toxin through the body quite generally, 

 whereas the pure toxin is chiefly held at or near the place of injection. 

 This diffusion tends to cause maximum antibody formation over an 

 entire portion of the body by a relatively small amount of free or easily 

 dissociated toxin in the toxin-antitoxin mixture. Smith inclines to 

 the belief that a similar phenomenon of diffusion may occur with sensi- 

 tized dead bacteria. 



In addition, the specific immune serum may aid in the disintegration 

 of the bacterial cell, either through the attachment of a bacteriolytic 

 amboceptor that would tend to lyse the bacterium with a complement 

 of the tissues, or through a preliminary action of opsonin which prepared 

 the bacterium for ultimate destruction and liberation of antigenic 

 principles. 



Autogenous versus Stock Bacterial Vaccines. It may be stated in 



general that autogenous vaccines, i. e., those prepared from the pa- 



1 Ann. de 1'Inst. Pasteur, 1913, xxvii, 597. 



