44 



IMMUNE SERA 



discovery the agglutinins were regarded as strictly 

 specific, i.e., a serum derived, for example, from a 

 typhoid infection would agglutinate only typhoid 

 bacilli and no others. After a time, however, it was 

 found that such a serum would frequently aggluti- 

 nate somewhat related organisms, though not, 

 usually, to so high a degree. In other words, while 

 agglutinins may be nearly, if not quite, specific in 

 their action, a serum which produces agglutination 

 may be far from being so. 



The following examples will illustrate the point. 

 In a case of infection with paratyphoid bacilli, 

 type B, the bacilli of the infecting type B were 

 agglutinated 1:5700; typhoid bacilli, however, only 

 1:120, while paratyphoid bacilli type A were not 

 agglutinated at all. In a case of typhoid infection 

 an agglutination with a dilution of i : 40 was obtained 

 for paratyphoid type B, while typhoid bacilli were 

 agglutinated in a dilution of i : 300 and over. As a 

 rule the agglutination with the infecting agent is by 

 far the strongest, i.e. it proceeds even in high dilu- 

 tions, whereas other bacteria require a stronger 

 concentration. 



This phenomenon is known as group agglutina- 

 tion. The bacteria which are agglutinated by one 

 and the same serum need not necessarily be related, 

 although usually this is the case. Conversely, 

 microorganisms which, because of their morpho- 

 logical or other biological characteristics, are re- 

 garded as entirely identical or nearly so, are sharply 



