CHAPTER IX 

 THE PRECIPITINS 



AGGLUTININS are antibodies obtained by the injection of particu- 

 late substances, and have the property of causing these substances 

 to collect into clumps. In exactly the same way proteids in solu- 

 tion, when injected into suitable animals, bring about the formation 

 of another class of antibodies, which possess the power of clumping 

 the molecules of the proteid in a solution similar to that injected. 

 This manifests itself by the formation of a precipitate. After the 

 addition of the clear antiserum to the clear proteid solution, the 

 mixture becomes opalescent, and then opaque, and after a time a 

 precipitate is cast down, leaving a clear supernatant fluid. Hence 

 these substances are called precipitins, the substance with which 

 they form a precipitate, and which calls them into existence when 

 injected into a suitable animal (their antigen), being termed pre- 

 cipitable substance, or precipitogen, and the insoluble combination 

 of the two, precipitum. They are in all respects closely allied to 

 \ agglutinins, if not absolutely identical. The fact of their acting in 

 V a clear fluid does not prove that they exert their effect in a true 

 solution, since modern physico-chemical research has shown that 

 proteids do not form solutions, but merely emulsions or suspensions 

 of molecules or of complexes of molecules. The effect of the 

 addition of a precipitin is to cause an agglutination of these 

 molecules, which is entirely analogous with the agglutination of 

 typhoid bacilli. The laws which govern the reactions of the 

 precipitins and agglutinins are entirely similar, and, theoretically, 

 it would probably be more accurate to consider them under one 

 head. The practical applications of the two classes of antibodies 

 are, however, very different, and it is more convenient to treat 

 them as separate substances. 



The first substances of this group to be discovered were the 

 bacterio - precipitins of Kraus, first investigated in 1897, anc ^ 

 referred to elsewhere. Kraus found that, if he took an old 



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