160 IMMUNE SERA 



over, as Ehrlich points out, even if such receptors 

 are present, it is possible for the animal to be immune 

 provided the receptors are situated only in indif- 

 ferent, vitally unimportant tissues. In some of 

 the lower animals there is reason to believe that the 

 toxin of tetanus does combine with such tissue 

 (Metchnikoff). 



Acquired Immunity, This may be either active 

 or passive, and either form may be acquired naturally 

 or artificially. As examples of naturally acquired 

 active immunity w r e may mention the immunity 

 developed by one attack of small-pox, scarlet 

 fever, etc. The immunity against small-pox con- 

 ferred by vaccination is an example of artificially 

 acquired active immunity; so is the preventive 

 inoculation with bacterial vaccine against typhoid 

 fever. The best illustration of artificially acquired 

 passive immunity is the injection of diphtheria 

 antitoxin into humans, while the transmission of 

 antitoxic immunity from mother to offspring is 

 an example of naturally acquired passive im- 

 munity. 



So far as maternal transmission of immunity is 

 concerned, a number of writers, among whom may 

 be mentioned Ehrlich, 1 Anderson, 2 and Theobald 



1 See Morgenroth's article in Kolle and Wasserman's Hand- 

 buch, Vol. iv, p. 784. 



2 Anderson, Bull. Hyg. Lab. U. S. Pub. Health and Mar. 

 Hosp. Serv., No. 30. 



