68 INFECTION AND RESISTANCE 



In speaking of this subject it is well to mention recent ob- 

 servations upon immunization with "sensitized" bacteria, 28 although 

 this necessitates anticipatory reference to subjects not so far dis- 

 cussed. It is a matter of common experience in laboratories that 

 rabbits and other animals will withstand relatively large amounts of 

 pathogenic bacteria if these are first treated with heated specific 

 immune serum (sensitized). This is probably due to the fact that 

 such "sensitized" micro-organisms are very rapidly taken up by 

 phagocytes. In spite of the phagocytosis, immunity is developed. 

 MetchnikofF and Besredka, in the communication alluded to above, 

 state that typhoid vaccination with unaltered living bacilli is efficient, 

 but is attended by severe local and general reactions. If the living 

 bacilli are first "sensitized" no such severe reaction occurs and im- 

 munization is nevertheless successful. The recent work of Gay points 

 in the same direction, and it is at least possible that by the practice of 

 sensitization we may be able to employ living unattenuated organisms 

 for purposes of immunization more extensively than we have in the 

 past. 



III. ACTIVE IMMUNIZATION WITH DEAD BACTERIA, AND BACTERIAL 



EXTRACTS 



This method is the one most extensively practiced in the labora- 

 tory immunization of animals. It is usual in most experiments of 

 this kind to inject dead organisms once or twice before living bac- 

 teria are administered. High degrees of resistance can in some 

 instances be attained by progressively increasing doses of dead cul- 

 tures only. This method is not only useful in experimental work, 

 but is clinically employed in the active immunization of human 

 beings as introduced by Wright and as applied, before Wright, to 

 tuberculosis (tuberculin treatment). But it is very probable that 

 "the immunity so attained is not entirely comparable to the immunity 

 following an attack of a disease, nor even that produced by the in- 

 jection of living bacteria. 



The method employed for killing the bacteria is of considerable 

 importance since, both by excessive heating as well as by too vigorous 

 chemical treatment, the immunizing properties of the bacterial 

 protein may be destroyed. In employing heat it is a safe rule never 

 to expose the bacteria for prolonged periods to temperatures which 

 considerably exceed the thermal death point. As a rule, heating non- 

 spore-forming bacteria to a temperature of from 65 to 70 C. for 



28 Refer to p. 159 and the discussion of the conception of "sensitization" 

 which follows. 



