THE PHENOMENON OF AGGLUTINATION 229 



tinable nor absorbed agglutinin, may have taken up altered agglu- 

 tinin or agglutinoid. We will have occasion to recur to this problem 

 in connection with our discussions of the capsulated bacteria and of 

 virulence. The explanations given above do not seem on the whole 

 satisfactory, and the problem is an exceedingly complex one. It has 

 been found indeed that the acquired resistance of bacteria against 

 agglutinins is not at all unique, and that acquired resistance against 

 serum lysins may be observed. 35 The extensive investigations of 

 Bail, Walker, 36 and others, on the nature of changes in virulence in 

 many invasive bacteria, and the knowledge more recently gained on 

 the resistance to phagocytosis of virulent strains of streptococci and 

 pneumococci are facts closely related in underlying principle to the 

 inagglutinability of typhoid strains cultivated in immune sera. 



That no two strains of bacteria of the same species are exactly 

 similar in their agglutinability in the same serum, moreover, is an 

 observation w r hich is made by every one who is in a position to carry 

 out routine Widal tests in hospital practice. The spontaneous ag- 

 glutination which occasionally occurs in the broth cultures of typhoid 

 bacilli used for this test in many laboratories 37 may often be re- 

 ferred, at least in the cases which have come to the writer's notice, to 

 an excessive acidity of the broth, a phenomenon which will be dis- 

 cussed in a subsequent paragraph. As far as the phenomenon of 

 variable agglutinability inherent in different strains is concerned, 

 however, it is of great practical importance in carrying out routine 

 Widal tests to bear this in mind and to control the strain of typhoid 

 bacilli employed in the reactions from this point of view. A strain 

 also which has been in use for a long time should be titrated with 

 agglutinating animal sera from time to time to determine whether or 

 not alterations in agglutinability have occurred. 



That the reaction of bacterial agglutination was specific was 

 noted, we have seen, by Gruber and Durham from the very begin- 

 ning. The closer study of the reaction in its application to bacterial 

 identification has led to interesting data which have revealed certain 

 definite limitations of this specificity. It has been found, for in- 

 stance, that, while immunization with any given species of bacteria 

 gives rise to a very marked increase of agglutinins for this species, 

 there are formed at the same time, though to a lesser degree, agglu- 

 tinins for bacteria of other species. This has been referred to as 

 "group reaction/' and the agglutinins appearing in such sera are 

 spoken of by German observers as "Haupt Aggluiinine" and "Neben" 

 or "Mit Agglutinine" In English texts they are usually referred to 

 as "chief" or "major" agglutinins and "para" or "minor" agglu- 

 tinins. Although, as a general rule, such group-agglutinin formation 



35 Eisenberg. Centralbl. f. Bakt., Vol. 34, p. 739, 1903. 



36 Walker. Centralbl. f. Bakt., Vol. 33, 1903. 



37 See section on Aggressins. 



