42G Major P. S. Leiean [April 23, 



of protective substances ready-made. Its value is indicated by tke 

 fact that the use of anti-diphtheritic serum has reduced the mortality 

 of diphtheria from 50 per cent, to 5 per cent. — which, in the 

 United States alone, means an annual saving of some 30,000 lives. 

 Unfortunately, for various technical reasons, this method is of only 

 very limited applicability. 



Active immunization, on the other hand, is effected by making 

 the body manufacture its own protective substances, which are semi- 

 permanent and prophylactic. The means adopted is that of injecting 

 the stimulating material which produces this effect in the process of 

 natural immunization occurring during an attack of the disease 

 itself. 



What measure of success has attended our efforts will be indicated 

 later, but first it is necessary to study in detail the exact process of 

 immunization as effected by natural means in the course of recovery 

 from, and subsequent protection against, bacterial infections. 



The Mechanism of Naturally Acquired Immunity. 



A sound grasp of this complex subject can only be obtained by a 

 review of the properties and inter-relationships of the various bodies 

 which take part in the production of immunity. 



In endeavouring to present this difficult subject in a manner 

 which is both simple and comprehensive, one is faced by three main 

 difficulties. The first is that of the starting-point — and, as lack of 

 interest in bacteriology may have led to certain of my audience not 

 having considered these matters previously, it appears desirable to 

 err on the safe side and deal with relevant factors, however simple. 



Next, it is difficult to illustrate all points by the details of any 

 single investigation or process. It is advisal)le, therefore, to present 

 the facts as far as possible dovetailed into one mental picture, with- 

 out the distraction of breaking off constantly to explain that one 

 or other fragment of the mosaic is derived from data relating to 

 some special infective process. 



And, lastly, it is impossible to avoid the use of technical terms, 

 because any research which is so advanced as this must of necessity 

 possess its own nomenclature. Use of technical terms is therefore 

 inevitable, and not due to emulation of the example of the gentleman 

 who would only refer to a bell as "a scintillating tintinabulator." 

 One rather sympathizes with the coster who, when reproached con- 

 cerning his language, remarked pathetically, " There's no other words 

 for it." 



In introducing the dramatis jJersonse it is advisable to take first 

 those which are normally present in the body, and, secondly, those 

 which only appear upon the stage after a bacterial invasion has 

 occurred . 



