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26 PASSIVE IMMUNITY 



PASSIVE IMMUNITY, the second form of acquired immunity, is 

 conferred by injecting into a susceptible animal the serum of one 

 which has acquired an active immunity to the disease in question. 

 It is a kind of second-hand immunity, acquired in virtue of the 

 reception of substances actively formed by another animal which 

 has had to fight against the infecting agent in order to form them. 

 In its production there is no necessary illness, however slight. 

 Such may occur, it is true, but it is not more than would be pro- 

 duced by normal serum, and stands in no necessary relationship 

 to the development of the immunity. And when such illness does 

 occur, it does so after the production of the immunity, and may 

 be very severe when the protection given is but slight, and 

 vice versa. 



For the production of passive immunity it is necessary to 

 inject the serum of an animal which has been artificially immun- 

 ized, that from one which is naturally immune being devoid 

 of action in this respect. To this general rule there are 

 one or two exceptions, which are perhaps more apparent than 

 real. 



Passive immunity is sometimes called antitoxic. The term, 

 however, is not a good one, since there are several varieties of 

 passive immunity, only one of which is due to an antitoxin. 



Passive immunity is specific that is, the serum of an animal 

 which has acquired immunity against one organism will protect 

 a second against that, and against no other. In this, of course, 

 it resembles active immunity, but the two differ in several 

 important particulars. 



i. As regards its production. Active immunity takes some 

 time usually a week or so to develop, dating from the infection 

 or injection of the vaccine, and in many cases at least its 

 appearance is preceded by a negative phase, in which the natural 

 immunity to the organism in question is lowered. But passive 

 immunity is established as soon as the serum has become mixed 

 with the blood of the person or animal injected, and there is no 

 negative phase. 



Hence in severe infections our best hope in the way of specific 

 medication is in the production of passive immunity. It is but 

 recently that the injection of vaccines was thought of in face of 

 an infection already developed, and it is obvious that the method 

 will be useless or dangerous in very severe and rapid infective 

 processes. Passive immunity, on the other hand, can be induced 



