AGGLUTININS 39 



serum of agglutinoids. These are assumed to 

 possess a higher affinity for the bacteria than do 

 the agglutinins and so prevent the latter from 

 acting on the bacteria. Since, however, the agglu- 

 tinins are usually far more abundant than the 

 agglutinoids, dilution of the serum dilutes the latter 

 to practically nothing, thus allowing the agglutinins, 

 to combine with the bacteria. Some recent experi- 

 ments by Field show that the pro zone* may have 

 an entirely different explanation, based on behavior 

 of bacteria and agglutinin as colloids. It has 

 already been stated that the union of agglutinin 

 and bacterium does not precipitate because there 

 is still sufficient electric potential ; the combination 

 carries a negative charge. Field believes that with 

 very large amounts of agglutinin (as in the pro 

 zone] the bacteria load themselves with so much 

 agglutinin that the combination now carries a 

 considerable positive charge. The surface tension 

 therefore is not sufficient to cause a clumping to 

 occur. Naturally the presence of salts does not 

 alter the condition as the kations also carry a 

 positive charge. 



Group Agglutinins. For some time after their 

 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- 



