4 o IMMUNE SERA 



and so prevent the latter from acting on the bacteria. 

 Since, however, the agglutinins are usually far more 

 abundant than the agglutinoids, dilution of the serum 

 dilutes the latter to practically nothing, thus allow- 

 ing the agglutinins to combine with the bacteria. 



Ehrlich's conception of the structure of the agglu- 

 tinin molecule and his views on the nature of the 

 agglutination reaction have been sharply combated. 



Elser very properly points out that not enough 

 attention has been paid to the effect of heat on serum, 

 and that alterations in the physical characters of the 

 serum may be sufficient to account for phenomena 

 heretofore ascribed to chemical changes. Among 

 other things he cites the effect of heat on horse serum ; 

 heating produces a marked increase in the viscosity 

 of the serum. It is obvious, therefore, when heated 

 sera are used in agglutination experiments, that this 

 purely physical characteristic exerts a profound in- 

 fluence on the result of the reaction. Differences in 

 the behavior of an agglutinating serum before and 

 after heating must therefore be interpreted with great 

 caution, and must not at once be taken to indicate 

 the chemical alteration of the agglutinin complex. 



Bordet, for example, cites an interesting experi- 

 ment of Gengou. An aqueous solution of agar, 

 so diluted as to be only slightly viscous at room 

 temperature, agglutinates barium sulphate sus- 

 pended in water. Heating such a solution destroys 

 this property without affecting the adsorbing 

 property; under these conditions it produces the 



