346 INFECTION AND RESISTANCE 



In essence also Pasteur's method of active immunization in rabies 

 represents such prophylactic vaccination, since, in this case, although 

 treatment is begun after infection has taken place, nevertheless the 

 process of immunization is carried out during the incubation period 

 before active manifestations of the disease have set in. Prophylactic 

 vaccination, therefore, is a valuable procedure which has reaped 

 remarkable results of recent years, especially in protection against 

 typhoid fever. In a subsequent section this phase of vaccination 

 is more extensively discussed, and we may therefore leave it for the 

 present. 



In this place we are more particularly concerned with the prob- 

 lem of the treatment of existing disease with vaccines prepared 

 from the bacteria by which the disease is caused. In how far this is 

 justifiable or even logical is a question which depends upon the con- 

 ditions of each individual case. We can approach the problem best 

 by roughly classifying the various forms in which infection occurs 

 in the human being. 



When bacteria gain entrance into the tissues of the human body, 

 granted that the organisms are pathogenic, an immediate struggle 

 ensues between the offensive properties of the micro-organisms and 

 the defensive powers of the tissues. The factors which determine 

 the outcome of such a combat have been more fully considered in 

 Chapter I. Briefly, if the defensive powers of the body greatly 

 preponderate the result is localization and rapid destruction of the 

 micro-organisms with cure. In such a case any form of treatment 

 is unnecessary. On the other hand, the balance of power may be 

 turned in the opposite direction, in which case the infectious process 

 becomes rapidly generalized, the bacteria enter the blood stream 

 and lymphatics, and the defensive powers are overwhelmed. In, 

 such a case also active immunization with vaccines is entirely use- 

 less. 



There are cases, however, in which the struggle is a more equal 

 one, and in which the infectious process is held in check by the 

 defenses, so that it takes a slow, chronic, localized form, and spreads, 

 if at all, very slowly. What is it in such a case that prevents com- 

 plete healing of the process ? The answer to this may be found both 

 in local and in systemic causes. Locally the lesion, after the pre- 

 liminary skirmishes, may become encapsulated either by fibrin 

 formation, clot, or other tissue changes so that, as Wright suggests, 

 the fluid constituents of the blood-plasma cannot easily approach the 

 organisms in the lesion. The same effect may result from internal 

 pressure by fluid and possibly by the presence of considerable quan- 

 tities of tissue detritus, by which protective serum constituents are 

 fixed and thus diverted from the bacteria. Against these factors, of 

 course, no form of immunization can be of value. Wright recog- 

 nizes this, and suggests the use of surgical evacuation, Bier's method, 



