224 MICROBES AND TOXINS 



the innumerable molecular' groups postulated by the side-chain 

 theory. For example, the antiserum prepared by an animal 

 against a protein has not the same properties as the serum 

 obtained against the same substance previously subjected to 

 heat (exp. of Obermayer and Pick). Hens' serum agglutinates 

 the lipoids extracted from the red corpuscles of the rabbit 

 much more vigorously than those from the ox. It is easy to 

 found a theory for specificity on the absorption phenomena, 

 and such theories already exist ; hitherto they have been too 

 philosophical, but it is satisfactory to know that experiment 

 has already furnished the germ of a scientific explanation. 



The Colloids After Bordet's explanations of agglutination 



and haemolysis in terms of molecular attractions and cohesions, 

 and in the light of his comparison of these phenomena to 

 dyeing processes, Zangger, Landsteiner, and Jagic established 

 experimentally the first analogies between immunity phenomena 

 and the physics of colloids. 



Reactions between colloids or between colloids and true 

 solutions can be reduced to phenomena of molecular attrac- 

 tion, of absorption, and adsorption. The bodies participate 

 in the reactions in variable proportions, influenced by tem- 

 perature and pressure. Colloids of opposite electrical charge 

 exert on each other a " flocking " or precipitating action, 

 which may be^masked when one or other is in excess in the 

 mixture ; one colloid may inhibit the precipitation of another 

 colloid by a, salt. In many cases, the law of the opposite 

 electrical charge may be masked by the fact that the proteins 

 are amphoteric colloids, and may neutralize acids and alkalies 

 equally well, behaving in acid solution as bases, in basic 

 solutions as acids. 



Are the antigens and antibodies of immunity colloids ? 

 The only ones whose chemical composition is known, namely, 

 the lipoids (fatty bodies typified by lecithin or cholesterin), 

 behave, in watery suspension, like the colloids ; the others, 

 which probably belong to the proteins, behave like colloids 

 from the point of view of diffusion in dialysis, heat, and 

 instability, and their principal reactions are closely analogous 



