Acquired immunity against micro-organisms 257 



the swelling of the bacterial membrane which becomes viscous and so 

 leads to the cohesion of the bacteria and the formation of clumps. 

 Thus transformed and rendered motionless, the bacteria succumb more 

 readily to the destructive action of the alexine. It is supposed that 

 the phagocytes do not intervene at all in these cases of acquired 

 immunity, except in a purely secondary fashion when they ingest 

 the bacteria already greatly weakened by the united action of the 

 agglutinin and the alexine. The principal rdle in this theory of 

 immunity is thus given to the agglutinative substance, which is re- 

 garded as being a microbial product, modified by the macrophages 

 and thrown into the blood. 



The discovery of this agglutination of bacteria has acquired great 

 importance, especially in connection with its application to the dia- 

 gnosis of typhoid fever. Widal 1 succeeded in showing that typhoid 

 bacilli agglutinate readily under the influence of blood serum and 

 other fluids (milk, transudations, tears, etc.) derived from patients 

 suffering from typhoid fever. As this phenomenon could be utilised 

 for the early recognition of the disease, it began to be studied with 

 great care and many interesting data concerning it have been col- 

 lected. The general outcome of these researches accords with the 

 conclusions drawn by Widal, and the serum-diagnosis of typhoid 

 fever has taken an important place among the methods used for 

 the recognition of this disease. This aspect of the question, however, 

 does not interest us from the point of view of the problem of im- 

 munity which we now have under consideration, and we cannot here 

 enter upon the study of the serum-diagnosis of typhoid fever and 

 certain other diseases (cholera, tuberculosis, pneumonia). Moreover, [271] 

 we must refrain from any analysis of the hypotheses advanced to 

 explain the mechanism of agglutination. A lively discussion has been 

 carried on between the partisans of the chemical theory according 

 to whom the agglutinin acts directly on the agglutinable substance 

 of the bacteria and the advocates of the physical theory, led by 

 Bordet 2 , who attribute the agglutination to modifications in the 

 molecular attractions which unite the agglutinable elements, be it 

 between each other or with the surrounding fluid. At one time 

 it was thought that Roger's 8 observation that the cell membranes 

 of Outturn cdbicans, when cultivated in the specific serum of 



1 BM. Soc. med. d. hop., Paris, 1896, 26 juin [Semaine med., Paris, 1896, p. 259]. 



2 Ann. de VInst. Pasteur, Paris, 1899, t. xin, p. 225. 



3 Rev. gin. d. sc. pures et appliq., Paris, 1 896, t. vii, p. 770. 



B. 17 



