AGGLUTININS 6l 



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 

 differentiated by means of their agglutination. 

 Because of this lack of absolute specificity the serum 

 diagnosis of infection or the identification of bac- 

 teria by means of agglutination tests, has value 

 only when very carefully tested. We have said 

 above that while agglutinins are specific, a serum 

 which produces agglutination may be far from 

 being so. The reason for this is that the serum 

 may contain several agglutinins. In fact, when 

 immunizing an animal with a particular bacterium 

 both specific and group agglutinins are produced. 

 This will perhaps be made clearer by reference to 

 the following diagram. We assume that the typhoid 

 bacillus possesses considerable protoplasm A, which 

 is specific for the typhoid bacillus; that it possesses 

 also certain protoplasm B, which is common to it, 



