DIFFERENTIATING BETWEEN A MIXED AND SINGLE INFECTION 285 



tides. Bordet cites the precipitation of clay as an analogous case: if 

 a little salt is added to a fine emulsion of potters' clay in distilled water, 

 the clay immediately clumps and falls to the bottom, the resemblance 

 between these flakes and the clump of agglutinated bacteria being very 

 striking. 



Specificity of Agglutinins. For a time after their discovery the ag- 

 glutinins were regarded as strictly specific, i. e., a typhoid-immune serum 

 would agglutinate only typhoid bacilli and no others. Gruber early 

 pointed out that an immune serum will frequently agglutinate other 

 closely related organisms, although not usually to so high a degree. 



Group or partial agglutinins, therefore, refer to the presence in a serum 

 of certain agglutinins that agglutinate certain other microorganisms 

 that are morphologically, biologically, and often pathogenetically 

 closely related to the homologous microorganism (the bacterium causing 

 the infection or used in artificial immunization). For example, a ty- 

 phoid-immune serum possesses, besides its greatly increased aggluti- 

 nating power for Bacillus typnosus, some agglutinin for Bacillus para- 

 typhosus, notably above that of normal serum. This is explained by the 

 very close biologic relationship of these bacteria, together with the fact 

 that the agglutinin-producing substance (agglutinogen) is a complex 

 and not a single chemical substance. This has been explained by Dur- 

 ham in the following example : If the typhoid agglutinogen is composed 

 of various elements, A, B, C, D, it is conceivable that the closely related 

 paratyphoids might contain one or more of these four agglutinogens, 

 and, therefore, the agglutinating power of the typhoid serum for a para- 

 typhoid bacillus, though not so great as on the typhoid bacillus, is still 

 considerable. Accordingly, in an infection with one microorganism 

 a specific agglutinin will be formed for that microorganism, and group 

 agglutinins for other more or less allied microorganisms, and conse- 

 quently the specificity of the agglutinating reaction depends upon the 

 principle of dilution, the specific agglutinin being present in largest 

 amount and operative in dilutions above the range of the group agglu- 

 tinins. 



Absorption Methods for Differentiating Between a Mixed and a 

 Single Infection. In 1902 Castellani discovered that if the serum of an 

 animal immunized against a certain microorganism contains that 

 microorganism in large numbers, the serum will lose its agglutinating 

 power not only for that microorganism, but also for all other varieties 

 on which it formerly acted. If, however, the serum contains the organ- 

 ism corresponding to the group agglutinins, the agglutinating power of 

 the serum for the homologous organism is reduced but little or not at all. 



