8 IMMUNE SERA 



able to demonstrate the transmission of immunity 

 to swine plague to the nursing young. 



In contrasting active with passive immunization 

 we may say that the former is usually more effect- 

 ive, more lasting, and productive of a general 

 immunity and not merely of one particular kind. 

 It is, however, sometimes difficult to carry out, 

 may involve some risk to the patient, and takes 

 time. Passive immunization, on the other hand, 

 is usually productive of only a limited kind of 

 immunity, i.e., antitoxic, bactericidal, opsonic, etc., 

 and therefore is often ineffective. Consisting, as 

 it usually does, in the injection of an alien serum, 

 passive immunization produces an immunity of but 

 short duration, the body apparently getting rid of 

 the alien proteid as rapidly as possible. It is well 

 known, as Park and others have shown, that anti- 

 toxin and other antibodies produced in an animal 

 disappear more rapidly when introduced into the 

 blood of another species than .when introduced 

 into one of the same species. In our experiments 

 with guinea pigs we have found that the homologous 

 antitoxin was retained in appreciable amounts for 

 at least six months while the heterologous anti- 

 bodies were noticeable to the same extent for only 

 four weeks. The great advantage of this form of 

 immunization, however, is its convenience, freedom 

 from risk to the patient, and above all, the fact 

 that the immunity is produced instantaneously. 



