THE MECHANISM OF AGGLUTINATION. 149 



precipitate that the agglutinin for red blood cells does to the 

 agglutinin for bacteria. It may be shown experimentally that 

 in the case of blood these precipitates not only are not indispen- 

 sable for the occurrence of strong agglutination but have no definite 

 relation to it. As we have already mentioned, the serum of a rabbit 

 that has been repeatedly treated with hen blood has the property 

 of agglutinating and dissolving hen corpuscles and of precipitating 

 hen serum. It also forms a precipitate with pigeon serum. We 

 therefore might expect that this serum would also agglutinate 

 pigeon corpuscles; as a matter of fact it has no more effect on these 

 corpuscles than does normal rabbit serum, which when mixed with 

 hen serum remains quite limpid. A slight agglutination, to be sure, 

 does occur either with the normal or specific rabbit serum, but it 

 is not so much as, for example, the agglutination of rabbit cor- 

 puscles by normal hen serum, in which latter instance also no 

 precipitate occurs. Guinea-pigs which have been given several 

 injections of defibrinated rabbit blood furnish a serum that has a 

 very intense clumping power for rabbit corpuscles, but which pro- 

 duces no clouding with rabbit serum. In other words there is no 

 necessary parallel between precipitate formation and an intense 

 clumping power, and any opinion that regards the formation of 

 such precipitates as the sine qua non of agglutination would seem 

 to be no longer tenable. 



Let us now consider Gruber's hypothesis which from the very 

 beginning has been extremely open to criticism. It is easy enough 

 to conceive that a sticky substance coming from the covering of 

 the bacteria should hold the micro-organisms together, but it is not 

 so easy to understand why it should bring these organisms together. 

 No evidence of the morphological modification which this hypothe- 

 sis implies has been found by Pfeiffer or ourselves on microscopical 

 examination of either living or stained preparations of bacteria or 

 red blood cells. Trumpp * thought that he did find an alteration 

 in agglutinated vibrios, but the bacteria in which he noted this 

 swelling had been in fluids containing not only agglutinin, but alexin 

 (or lysin), that is, the bactericidal substance which is destroyed 

 by a temperature of 55 degrees. This alexin is very destructive 

 both for vibrios and for red blood cells; it dissolves the latter and 

 * Archiv fur Hygiene, 1898. 



