THE PHENOMENON OF PRECIPITATION 265 



prevent their subsequent precipitation by active precipitin. An 

 illustration of this is found in the following protocol taken from the 

 paper by Kraus and v. Pirquet (loc. cit., p. 69). 



(a) 5 c. c. cholera filtrate + 0.5 c. c. inactiv. (60) cholera serum = no precipitate 



after 10 hours at 37 C. 

 After 10 hours add 0.5 c. c. active cholera serum = no precipitate. 



(b) Omitted. 



(c) Omitted. 



(d) 5 c. c. cholera filtrate + 0.5 c. c. active cholera serum = after 10 hrs. 



typical precipitate. 



From this it was concluded that heat may destroy the zymophore 

 or coagulating group of precipitins, leading to the formation of 

 "precipitinoids" which, like agglutinoids, may have a higher affinity 

 for the antigen than is possessed by the uninjured antibody. 



Subsequently there were opposed to these views the physical in- 

 terpretations which have been outlined sufficiently in the section on 

 Agglutination (see p. 240). In the case of precipitation the anal- 

 ogy between colloidal reactions and the serum phenomena is fully as 

 striking as in the former, an analogy in the delineation of which the 

 first credit belongs to Landsteiner, 44 45 and important further contri- 

 butions have been made by Neisser and Friedemann, Forges, Gen- 

 gou, and a number of others. As in agglutination and colloidal floc- 

 culation, the presence of salts (electrolytes) fundamentally influ- 

 ences the occurrence of precipitin reactions; and in both colloidal 

 and precipitin reactions the relative concentration of the reacting 

 bodies is paramount in determining whether or not precipitation 

 takes place. In this connection the most frequently observed inhibi- 

 tion occurring in serum precipitations is that which is caused by an 

 excess of antigen. An example of this is as follows : 



This is entirely analogous to the inhibition which may occur 

 when, let us say, a weak gelatin solution is added to a colloidal sus- 

 pension of arsenic trisulphid ; or blood serum is added to mastic or 

 arsenic suspensions. In both cases inhibition zones appear which 



44 Landsteiner and Jagic. Munch, med, Woch., No. 18, 1903; No. 27, 

 1904; Wien. kl. Woch., No. 3, 1904. 



45 Landsteiner and Stankovic. Centralbl. f. Bakt., Vols. 41 and 42, 1906. 



