AGGLUTINATION. 485 



serum produced a uniform turbidity. Gruber and Durham, in 

 investigating Pfeiffer's reaction, discovered an analogous phe- 

 nomenon. They found that when a small quantity of the serum 

 of an animal highly immunised against a particular motile 

 organism (cholera vibrio, typhoid bacillus, etc.) is added to an 

 emulsion of the organisms, the latter lose their motility and 

 become agglutinated into clumps. In a small test-tube a reac- 

 tion in this way occurs which is visible to the naked eye, a sort 

 of precipitate forming which consists of masses of the bacteria. 

 Non-motile organisms also may be agglutinated by the corre- 

 sponding serum, as may also red corpuscles by a hemolytic 

 serum. As a rule, the higher the degree of immunity the smaller 

 is the amount of serum necessary to produce agglutination. 

 The phenomenon depends upon the presence of definite bodies 

 in the serum called agglutinins. In each case these can only 

 clump a certain amount of bacteria, and are used up in the 

 process, apparently by a combination with the bacteria, probably 

 attended with a physical change in the envelopes of the latter, 

 and this Gruber and Durham consider forms the essential part 

 of Pfeiffer's reaction. 



The observations just described have led to the discovery 

 of the method of serum diagnosis of disease, which has been 

 applied especially to typhoid fever, as already detailed {vide 

 p. 340). It had been already found that the serum of conva- 

 lescents from typhoid fever could protect animals to a certain 

 extent against typhoid fever, and, in view of the facts experi- 

 mentally established, it appeared a natural proceeding to inquire 

 whether such serum possessed an agglutinative action and at 

 what stage of the disease it appeared. The result, obtained 

 independently by Griinbaum and Widal, but first published by 

 the latter, 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, bacillary dysentery, glanders, plague, infection by 

 Gaertner's bacillus, B. coli, etc. 



The physical changes on which agglutination depends cannot 

 as yet be said to be fully understood. ' As stated above, Gruber 

 and Durham considered that the agglutinin produced a change 



