486 IMMUNITY. 



in the envelope of the bacterium, causing it to swell up and 

 become viscous, and there are certain facts in favour of this view. 

 On the other hand, this is probably not the full explanation, as it 

 has been shown by Nicolle and by Kruse that if an old bacterial, 

 culture be filtered through porcelain, the addition of some of 

 the corresponding serum produces some change in it, so that 

 even minute inorganic particles become aggregated into clumps. 

 The phenomenon would thus appear to be the result of the 

 interaction of the agglutinin and some substance in the bacterial 

 cell (which substance evidently leads to the development of the 

 agglutinin in the living body) ; and, as Duclaux states, is closely 

 allied to a process of coagulation. Of greater importance, how- 

 ever, is the relationship of agglutination to immunity. Gruber 

 and Durham considered that agglutination was the essential 

 part of Pfeiffer's reaction or lysogenesis, the change produced in 

 the bacteria allowing the bactericidal action naturally possessed 

 by the serum to come into play. Others, again, consider that 

 the two are independent of one another. The fact that the 

 agglutinative power appears early in an infective disease is often 

 pointed to as proof of such a view. This line of reasoning is 

 not, however, by itself conclusive, as we must suppose that the 

 reaction, or series of reactions, leading to immunity begins at an 

 early period and gradually increases until cure results. It is 

 also to be noted that agglutinins accord with protective sub- 

 stances as regards resistance to heat {i.e. a serum heated to 55° C. 

 loses its bactericidal or lysogenic power while the agglutinative 

 and protective properties remain, vide p. 482). On the other 

 hand, a serum may be highly protective without being agglutina- 

 tive, and when the two properties are present together they do 

 not always run in the same proportion. It is doubtful, however, 

 whether a serum ever possesses a high degree of agglutination 

 without having some protective power. On the whole it seems 

 safe to say that agglutinins and immune-bodies, though not 

 necessarily identical, are the products of corresponding reactive 

 processes, and their formation is governed by corresponding laws. 

 The bacterial cell, containing as it does various complicated 

 organic constituents, may cause the formation of more than one 

 anti-substance, and each of these has a combining affinity for 

 part of the bacterial body. Agglutination is most probably to 

 be regarded as a phenomenon prejudicial to the corresponding 



