544 IMMUNITY 



ance to heat also varies when the serum is diluted with salt 

 solution, and it has been shown that conditions which interfere 

 with the coagulation of the proteins increase their resistance. 

 Like antitoxins, agglutinins seem to be chiefly contained in the 

 globulin fraction. Discussion has taken place as to the relation 

 of agglutinins to immune-bodies and as to how far agglutination is 

 an indication of immunity. It may be said that in the case of 

 certain sera investigated it has been shown that the immune- 

 body and the agglutinin are separate substances, but it would 

 not be justifiable to say this is always the case. And while the 

 agglutinative power cannot in itself be taken as the measure of 

 the degree of immunity, agglutinins and immune-bodies are the 

 products of corresponding reactive processes, and their forma- 

 tion is governed by corresponding laws. Agglutinins become 

 fixed in definite proportion by the receptors of the bacteria that 

 is, the agglutinin becomes used up in the process of agglutination, 

 and it has been shown that bacteria may take up many times 

 the amount necessary to their agglutination a corresponding 

 fact to what has been established with regard to immune-bodies 

 of hsemolytic sera. The agglutinins are specific in the sense 

 which has been explained above (p. 521). It can be shown by 

 the method of absorption that in an agglutinating serum there 

 may be several agglutinins with different combining groups, some 

 of which may be taken up by organisms allied to that which 

 has given rise to the anti-serum. 



Besides those stated above, other phenomena have been 

 observed in the interaction of anti-sera and the corresponding 

 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 proportion of the homo- 

 logous 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. 



Precipitins. Shortly after the discovery of agglutinins, Kraus 

 showed in the case of the organisms of typhoid, cholera and 

 plague, that the anti-serum not only caused agglutination, but 

 when added to a filtrate of a culture of the corresponding 

 bacterium produced a cloudiness and afterwards a precipitate. 

 To the substance in the immune-serum which brought about 

 this effect he gave the name of precipitin. Subsequent study 

 has shown that this phenomenon is closely related to agglutina- 

 tion ; in fact several authorities consider that they represent the 



