IMMUNITY 249 



theria by rendering him artificially immune, but it acts 

 as a curative agent in neutralizing the poison of the disease 

 if applied in the earlier stages of the disease. 



The blood serum of animals hyper-immunized against hog 

 cholera or diphtheria varies greatly in the amount of anti- 

 1 todies formed. Before it is used it is necessary to know 

 somrthinir of the strength or potency of the serum, so that 

 the proper quantity to be used in the animal to be pro- 

 tected may he determined. This is accomplished, in the 

 ease of \\o<r cholera, by inoculating a number of young pigs 

 with a definite quantity of the disease virus and a varying 

 amount of the protective serum, noting the amount which 

 is required to protect the animal against the artificial in- 

 oculation. 



In the case of diphtheria and tetanus antitoxin a num- 

 ber of guinea-pigs are inoculated with a definite amount 

 of toxin and with varying amounts of antitoxin. If the 

 antitoxin administered to a particular individual is suffi- 

 cient to neutralize the toxin, the animal remains alive. If 

 the toxin is in excess, the animal dies. 



Persistence of immunity. Passive immunity, produced 

 by the transfer of the anti-bodies, is always of short dura- 

 tion as compared with the active immunity produced by ar- 

 tificial means, while the active immunity produced as a re- 

 sult of the natural cause of the disease persists for a still 

 longer period. The variation in time during which pro- 

 tection persists must be taken into account in the practical 

 employment of serums and vaccines in the prevention of an- 

 imal diseases. 



Exit of organisms from body. Almost without excep- 

 tion, the pathogenic organisms grow only in the bodies of 

 susceptible animals. Their continued existence in nature 

 is therefore dependent upon their expulsion from the dis- 

 eased body, and the opportunity for introduction into a 



