MANUFACTURE OF VACCINES 561 



materials such as serums, tuberculins and vaccines, it is difficult to 

 realize the various steps necessary in the production of a safe and active 

 product. The manipulations attending the preparation of the materials 

 require large equipment, expensive apparatus and the services of trained 

 laboratory experts. Animals which are used in the work must be 

 quarantined and carefully inspected before being placed under treat- 

 ment. The sanitary conditions of the laboratories, operation rooms 

 and stables must be of the best. 



Infection of the animal organism is due to absence of natural 01 

 acquired resistance. The natural resistant forces of the animal body 

 may be such that insusceptibiUty to specific microbial invasion is 

 present; such a condition is called natural immunity. Acquired im- 

 munity, 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 invasions. Acquired 

 immunity may be active or passive, and may be brought about by appli- 

 cation of a vaccine or an antiserum. The application of smallpox 

 vaccine causes a specific reaction in the body, stimulating the develop- 

 ment of natural defences against smallpox virus, and is followed by a 

 condition of active immunity which is relatively permanent in dura- 

 tion. The use of diphtheria antitoxin, which contains the antibodies 

 capable of neutralizing the diphtheria toxin molecules, results in passive 

 immunity and affords temporary protection. 



Actively Immunizing Substances (Vaccines) 



Vaccines* are essentially weakened or modified viruses. Such 

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

 as a rule, only on animals which are free from the specific disease 

 in question, because, theoretically, if a specific vaccine were applied to 

 a patient suffering from a given infectious disease, the introduction of 

 the attenuated organisms, or virus, would tend to augment the 

 virulence of the infection. The action of such vaccines is preventive 

 or prophylactic, and not curative. 



Attenuated Viruses. — There are several methods which may be 

 employed in attenuating or modifying viruses. The processes involve 



* The term "vaccine" is also loosely applied to suspensions containing killed microSrgan- 

 isms. 



36 



