PRECIPITINS 581 



Like immune-bodies, agglutinins are not destroyed at 55° 0. 

 (a temperature sufficient to annul bactericidal action), but 

 different agglutinins show variations in this respect, some being 

 affected by a temperature little above that named. The resist- 

 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 haemolytic sera. The agglutinins are specific in the sense 

 which has been explained above (p. 561). 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 (p. 390). 



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 propor- 

 tion of the homologous 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 

 whet 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 



