THE AGGLUTININS 205 



(if the emulsion is thick enough) all the organisms will be found to 

 have collected into a single mass or a few masses, the rest of the 

 fluid being quite clear. Finally, these masses will sink to the 

 bottom of the vessel, and it will be noted that if the bacilli in 

 the control specimen also sink (as happens with killed organisms), 

 the masses will be much more voluminous than the deposit of 

 unagglutinated bacteria. A microscopic examination of the de- 

 posit in the two cases will show why this is. In the deposit of 

 dead bacilli the separate rods have sunk down slowly, and have 

 packed themselves closely together, and will be found, to a very 

 large extent, to lie horizontally side by side. In the agglutinated 

 mass the bacilli point indifferently in all directions, and the explana- 

 tion suggests itself that they have been drawn forcibly together by 

 a centripetal force, and have not had time to adapt themselves so 

 as to take up as little room as possible. A result of this is that it 

 is easy to distinguish between a specimen that has agglutinated 

 and one in which the bacilli have simply settled, even although the 

 actual occurrence of the phenomenon has not been witnessed. 



The reaction is given with the serum of immunized animals, and 

 is a general one. It is given with nearly all species of bacteria, 

 though to a very different extent in different cases, with red blood- 

 corpuscles, leucocytes, and with cells of all kinds. The occurrence 

 of motility is not necessary for it, and dead bacilli will clump 

 almost or quite as well as living ones. The reaction is, in general, 

 specific, and a serum which is strongly agglutinating as regards 

 one species of organism may be entirely devoid of action on others. 

 Hence it was proposed by Gruber and Durham as a test for the 

 identification of bacteria, and is of great value. Thus, when a 

 bacteriologist has isolated a culture of an organism resembling 

 B. typhosus from a patient suspected of having typhoid fever, or 

 from a sample of water supposed to be contaminated, the first step 

 in the identification is made by observing whether it is clumped by 

 a serum known to have agglutinating powers over typhoid bacilli 

 and not over others. Other tests are necessary for its complete 

 identification, but these are slower, and for some purposes un- 

 necessary. The clinical diagnosis of cholera by means of cultures 

 from the stools is carried out in the same way, and sera adapted 

 for either purpose can be obtained commercially. 



The reaction, however, is not an absolutely specific one, and it 

 is found that a given immune serum may clump not only the 

 culture used in its production, but also those of closely allied 



