ANTISERA 41 



high therapeutic value both for protective and cura- 

 tive purposes. By their use in persons liable to be ex- 

 posed to an infection, protection for a limited period 

 of time may be conferred. This protection by passive 

 immunization is not so durable as by active immuniza- 

 tion, though the time necessary to acquire it is much 

 less, a fact of considerable importance in some cases. 

 It has been estimated, experimentally and clinically, 

 that the duration of passive immunity persists from 

 three to six weeks, while active immunization may 

 endure for a year or two. Moreover, studies in pas- 

 sive immunization have sho^vn that the duration of 

 immunity from homologous antisera lasts three to four 

 times as long as that from heterologous immune sera. 



Antitoxic serum in some infections has a high 

 curative value in persons already suffering from the 

 disease, and indeed in certain infections is the most 

 important means of treatment. In the case of other 

 diseases, the value of the antitoxic serum as a curative 

 agent is problematical. 



Antibacterial sera, in infections where they are of 

 any value at all, are indicated especially to overcome 

 the immediate infection, owing to their rapid effects, 

 while more lasting protection in this class of infections 

 is conferred by active immunization which pursues a 

 slower course. The indications for active and passive 

 immunization in specific cases will be taken up later. 



