468 MICROBIOLOGY OF SPECIAL INDUSTRIES. 



reduced to a minimum the danger formerly involved in the use of such 

 products and has materially increased their reliability. 



To one who is not a student of microbiology and preventive medicine, 

 or not familiar with the technic involved in preparing biological mate- 

 rials such as sera, tuberculin, and vaccines, it is difficult to realize the 

 various necessary steps in the production of a safe and active product. 

 The animals used in the work must be placed under quarantine and 

 carefully inspected before being placed under treatment. The sanitary 

 conditions of the stables, operating rooms and laboratories must be of the 

 best. The various manipulations attending the preparation of the 

 materials require large equipment, expensive apparatus and the services 

 of trained laboratory experts. The above precautions must be observed, 

 however, in order that resulting biological products may be thoroughly 

 reliable. 



i - Infection of the animal organism is due to absence of natural or 

 acquired resistance. The natural resistant forces of the animal body 

 may be such that insusceptibility to specific microbial invasion is present ; 

 such a condition is called natural immunity. Acquired immunity, on 

 the other hand, refers to a condition in which the natural susceptibility 

 of the animal body is replaced by a temporary or permanent resistance 

 toward specific microbial invasion. Acquired immunity may be 

 active or passive. For instance, acquired immunity may be brought 

 about by the application of a vaccine or an antitoxin. The application 

 of smallpox vaccine causes a reaction in the body, or a mild form of the 

 disease, and brings about a condition of active immunity which is relatively 

 permanent in duration. The use of diphtheria antitoxin, which contains 

 the theoretical antibodies capable of neutralizing the diphtheria toxin 

 molecules, results in passive immunity and affords relatively temporary 

 protection. 



ACTIVE IMMUNIZING SUBSTANCES. (VACCINES.) 



Vaccines are essentially weakened or modified viruses. Such materials 

 as smallpox, blackleg and anthrax vaccines may be used with safety, 

 as a rule, only on individuals who are free from the specific disease in 

 question, because, if a specific vaccine were applied to a patient suffering 

 from a given infectious disease, the introduction of the attenuated organ- 

 isms, or virus, would tend to accelerate the infection. The general action 

 of these vaccines is therefore preventive or prophylactic and not curative. 



