

CHAPTER XIV. 



IMMUNITY. 1 



THE PROPERTIES OF IMMUNE SERUMS. 



Introduction. The mechanism of immunity, p. 222. 

 Section I. Prophylactic and therapeutic serums, p. 223. 

 Section II. Antitoxins, p. 224. 

 Section III. Agglutinins, p. 225. 



The mechanism of agglutination, p. 226. 

 Section IV. Bactericidal properties, p. 227. 



The mechanism of bacteriolysis, p. 228. Haemolysins, p. 230. The mechanism of 

 haemolysis, p. 231. The fixation of the complement, p. 232. 

 Section V. Opsonins, p. 239. 



IMMUNITY as the word is applied in bacteriology denotes the faculty 

 possessed by a living animal of resisting an infection or intoxication. 



Immunity to a particular organism or toxin may be natural or acquired. 



Natural immunity is a function of the species and only rarely of the race. 

 In some cases it has a relation to age : thus, adults may be immune while 

 the young of the same species are susceptible to a particular infection or 

 intoxication. Again immunity may be absolute or relative. 



Acquired immunity to a specific disease may be a natural condition resulting 

 from an attack of that disease ; for instance, a person rarely suffers from 

 more than one attack of enteric fever, measles or anthrax ; or it may be a 

 condition artificially produced in an individual in response to the inoculation 

 of a virus, a toxin, or the serum of an immunized animal. 



Immunity artificially produced may be active or passive. 



Active immunity is the result of the inoculation of small doses of vigorous 

 cultures of living organisms, of cultures of living organisms attenuated either 

 by heat or by prolonged artificial cultivation, of dead organisms, or of the 

 toxins which organisms produce. An active reaction takes place in the living 

 tissues in response to the inoculation with the result that the subject has 

 acquired certain new properties and these will have to be studied in detail. 

 Active immunity is only acquired slowly and then at the cost of a real and 

 occasionally serious disease during which the tissues may be highly susceptible 

 to further inoculation of the particular virus ; but on the other hand the 



1 It would obviously be beyond the scope of a book such as this to enter into a detailed 

 study of immunity and the theories associated with it. The present chapter is therefore 

 limited to such explanations as are indispensable to the proper understanding of the 

 subsequent chapters and to an account of the principal methods of demonstrating the 

 properties of immune serums. 



