AGGLUTINATION. 483 



probably attended with a physical change in their envelopes, 

 and this they consider forms the essential part of Pfeiffer's 

 reaction. When the organisms are thus weakened by the 

 specific body, the bactericidal power of the normal 

 peritoneal fluid or blood serum, as the case may be, comes 

 into action and completes the process. 



The observations just described have led to the dis- 

 covery of the method of serum diagnosis of disease, which 

 has been applied especially to typhoid fever, as already 

 detailed (vide p. 343). It had been already found that the 

 serum of convalescents from typhoid fever could protect 

 animals to a certain extent against typhoid fever, and, in 

 view of the facts experimentally established, it appeared a 

 natural proceeding to enquire whether such serum possessed 

 an agglutinative action and at what stage of the disease it 

 appeared. The result, first published by Widal, was to 

 show that the serum possessed this specific action long 

 before the cure of the disease, in fact shortly after infection 

 had taken place. It is probable that it depends upon a 

 process of immunisation developing from an early stage of 

 the disease. Agglutination is also observed in the case of 

 cholera, Malta fever, glanders, infection by Gartner's bacillus, 

 B. coli, etc. 



Besides those stated above, other phenomena have been 

 observed in the inter-action of anti-sera and the correspond- 

 ing bacteria. For example, it has been shown that when 

 certain bacteria, e.g., the typhoid bacillus, B. coli and B. 

 proteus, are grown in bouillon containing a small propor- 

 tion of the homologous serum; their morphological characters 

 may be altered, growth taking place in the form of threads 

 or chains, which are not observed in ordinary conditions. 

 In other instances a serum may inhibit some of the vital 

 functions of the corresponding bacterium. 



When we come to enquire into the relation of the 

 various properties of an antimicrobic serum to one another, 

 we find that there still prevails considerable doubt. As 

 will be seen from the above, practically all observers are 

 agreed that in the destruction of a bacterium either within 



