THE PRECIPITINS 23! 



posed only to act on the proteid which has been used for the 

 immunization, whereas the partial ones act both on it and on 

 other foreign proteids. If these are formed at different rates of 

 speed, it is easy to see that at a certain stage in the process of 

 immunization an animal's serum may contain a precipitin (e.g., 

 the special precipitin) and a precipitable substance (e.g., that which 

 acts as an antigen to the partial precipitin). In most of their 

 reactions the precipitins certainly act as if they had a powerful 

 combining affinity for their antigens, and von Dungern's observa- 

 tions may supply the explanation of the apparent discrepancies. 



If precipitable substance (e.g., egg-albumin) be heated, it under- 

 goes a change comparable to that sustained by precipitin on 

 heating : it loses its power of becoming precipitated with anti- 

 serum, but retains its function of combining therewith. This 

 altered proteid is sometimes called precipitoid, since it is analogous 

 with the precipitoid derived from precipitin by the action of heat. 

 The term, however, is a bad one, since its use leads to confusion 

 between the two substances, and the only word which is at all 

 suitable is "precipitogenoid." Precipitogenoid, therefore, consists 

 of the haptophore portion of the molecule of precipitogen, another 

 portion of this molecule which must be present in order that the 

 reaction may take place being destroyed. From the fact that it 

 retains its haptophore group we should expect it to act as an 

 antigen, just as toxoid does ; and this is the case, for the injection 

 of precipitogenoid calls forth the production of ordinary precipitin. 

 In any complete quantitative study of the interactions of serum 

 and antiserum it is necessary to investigate the question of the 

 presence of precipitogenoid as well as of precipitoid, and the 

 problems thus may become complicated in the extreme. 



Precipitins, like the agglutinins, act most quickly at body 

 temperatures, and show their essential similarity in the fact that 

 salts are necessary for both reactions ; short of absolute absence, 

 the amount of the precipitum form depends on the quantity of 

 salts present (Friedmann). 



There is an interesting analogy between the agglutinins and the 

 precipitins, in that the latter as well as the former are occasionally 

 seen in the serum of unimmunized animals, though the precipitins 

 are much more uncommon in this situation. Thus, Hoke found 

 the presence of bacterio-precipitins to nitrates from B. typhosus 

 and V. cholera fairly frequently present in the serum of the ox, 

 more rarely in that of the goat, pig, and sheep. The presence of 



