158 STUDIES IN IMMUNITY. 



antiserum. This specific precipitating serum, moreover, is pro- 

 duced by treating animals much in the same way as to produce 

 agglutinating sera for bacteria. There are numerous evidences of 

 similarity between these different phenomena, so that one is forced 

 to accept a single general explanation for them all, and to attribute 

 the occurrence of agglutination to changes in molecular adhesion. 



If we were to choose the specific agglutination of bacteria as an 

 example of these phenomena we might say that the agglutinin which 

 unites with the bacteria acts by modifying the relations of molecular 

 attraction both between the individual bacterial particles and 

 between these particles and the surrounding fluid. The agglutinin 

 affects only a certain definite bacterial species. During the first 

 period of agglutination the individual constitution of the bacteria 

 in question is much in evidence. It is, perhaps, essential that bac- 

 teria, in order to be affected by an agglutinin, should be suffi- 

 ciently intact, as would seem indicated by certain of Malvoz's* 

 experiments. 



But as soon as the change in molecular adhesion has been produced 

 the bacteria collect as do inorganic particles. It is not necessary to 

 imagine that their structure has anything to do with it ; nor to think 

 that the bacteria must stick to one another as a label sticks to a 

 bottle, by means of some peculiar adhesive substance which covers 

 the cilia, or owing to a swollen outer membrane. This phenomenon 

 of the collection of particles by means of some influence which 

 changes their molecular attraction should by definition be placed 

 among the phenomena of coagulation as Duclaux has described 

 them. 



From this standpoint then the phenomenon of agglutination is 

 divided into two distinct phases. In the first phase the scattered 

 bacteria are affected by the agglutinin and absorb it. This causes 

 modifications in their properties of molecular adhesion. The 

 existence of these modifications during the second phase brings 

 about agglutination properly speaking. 



This division into two periods is neither artificial nor imaginary. 

 These two phases may indeed be separated and the first be brought about 

 without the second. The experiment already mentioned that shows 



* Recherches sur I'agglutination du bacille typhique. Annales de 1'Institut 

 Pasteur, July, 1897 



