48 IMMUNE SERA 



We are not yet informed as to the nature of these 

 phenomena. A number of theories have been ad- 

 vanced, into which, however, we cannot here enter. 



In some cases the agglutinins are active even in 

 very high dilutions. Thus in typhoid patients 

 and typhoid convalescents a distinct agglutination 

 has been observed in dilutions of i : 5000, and this 

 action persisted for years, though not, of course, 

 in the same degree. Even normal blood-serum, 

 when undiluted, often produces agglutination. But 

 the above specific agglutinins, which do not exist 

 beforehand, being formed only in consequence of 

 an infection, are characterized by this, that the 

 agglutination occurs even when the serum is diluted 

 (at least i : 30 to i .'50), and, furthermore, that after 

 this dilution the action is still specific, i.e. cholera 

 immune serum agglutinates only cholera bacilli, 

 typhoid immune serum only typhoid bacilli, etc. 

 This specificity, however, as will be shown later, 

 is not always absolute. 



This reaction is at present applied principally 

 to the diagnosis of typhoid fever and glanders 

 and also to differentiate between the several strains 

 of the various organisms. For example, the pneumo- 

 cocci have been found to fall into four well-defined 

 groups, only one of which when injected into suitable 

 animals produces a serum having curative powers. 

 It is, therefore, of the highest importance to identify 

 the particular type of organism found in a patient's 



