542 IMMUNITY 



phenomenon is not peculiar to bacteria ; it is seen, for example, 

 when an animal is injected with the red corpuscles of another 

 species, hcemagglutinins appearing in the serum, which have a 

 corresponding specificity. 



The physical changes on which agglutination depends cannot 

 as yet be said to be fully understood. Griiber and Durham 

 considered that the agglutinin produced a change in the envelope 

 of the bacterium, causing it to swell up and become viscous, but 

 the facts since established show that this is not the true explana- 

 tion. For example, it has been shown by Nicolle and by Kruse 

 that if an old bacterial culture be filtered through porcelain, the 

 addition of some of the corresponding anti-serum produces a 

 sort of granular precipitate in it; and that when, as in the 

 agglutination of bacteria, minute inorganic particles are added 

 to the mixture, they become aggregated into clumps. The 

 phenomenon would thus appear to be the result of the inter- 

 action of the agglutinin and some substance in the bacterial cell 

 which is known as the agglutinable substance or as the agglu- 

 tinogen. Joos has found in the case of the typhoid bacillus that 

 there are two agglutinable substances, which differ in their 

 resistance to heat a and ft agglutinogen, and that they give 

 rise to corresponding agglutinins. Further, as the result of a 

 comparative study of the agglutinins of a motile and a non- 

 motile variety of the hog cholera bacillus, Theobald Smith has 

 come to the conclusion that there is an agglutinin which is pro- 

 duced by and acts on the flagella and another which is similarly 

 related to the bacterial bodies. The former acts in very much 

 higher dilutions than the latter, and this is regarded as an ex- 

 planation of the fact that in the case of non-motile organisms 

 the agglutinating serum acts only in proportionately high con- 

 centration as compared with the case of most motile forms. 

 Another factor necessary for the phenomenon of agglutination 

 is a proper salt content. Bordet showed that if the clumps of 

 agglutinated bacteria are freed from salt by washing in distilled 

 water they become resolved, and that on the addition of some 

 sodium chloride they are formed again, and Joos has also brought 

 forward striking confirmatory evidence as to the necessity for 

 the presence of salts. It is thus probable that in the pheno- 

 menon of agglutination as ordinarily understood more than one 

 factor is concerned, and it is possible that in part it may depend 

 on some altered molecular relationship of the bacteria to the 

 surrounding fluid analogous to altered surface tension. 



In the phenomenon of agglutination we have to distinguish 

 two factors, namely, the combination of agglutinin and agglu- 



