H. M. POWELL 825 



protein material intended as antigen and the serum proteins of the animal being im- 

 munized, thus possibly constituting a sort of synthetic antigen. Also the criteria by 

 means of which the protein nature of a material is judged are very crude compared to 

 the delicacy of the biological reactions. In attempts which have been made toward 

 changing precipitinogen physically,' as by heating or chemically by the addition of 

 various chemical reagents, the changed properties, including those of specificity, seem 

 to be accounted for to a considerable extent by the fact that many precipitinogens are 

 complex materials, in many cases mixtures of several components, and that the various 

 procedures resorted to act as a sorting device through which only some components 

 can pass. Prezone phenomena also are involved as well as the rapidity of production 

 of antibody in the immunized animal. Where questions of altered specificity are in- 

 volved it is necessary to carry out immunization procedures for a longer period of 

 time, for altered antigen-incited antibody may then be found to give reactions with 

 unchanged native antigen. Also significance is to be attached to the failure to obtain 

 iso-precipitins against altered precipitinogen. It appears very desirable to obtain 

 greater knowledge of the characteristics of precipitinogens which can safely be 

 regarded as single individual substances. Lack of ample accurate knowledge in this 

 respect is probably not common to precipitinogen-precipitin study alone but rather 

 is the case in several fields of immunology,^ 



PRECIPITIN 



The properties of precipitins are like those of the agglutinins. They are fairly 

 heat-resistant antibodies present for the most part in the serum of immune animals 

 only. Their activity does not depend on the presence of complement, and when once 

 inactivated at temperatures of about 70° C, they cannot be reactivated. Their potency 

 can be most readily estimated by testing the immune serum undiluted against a series 

 of dilutions of precipitinogen. Accurate measurement necessitates learning both anti- 

 gen and antibody titres through testing a series of dilutions of each, and determining 

 the points of maximum precipitation. 



The manner in which precipitins act upon homologous antigens is not clearly under- 

 stood. Considerable species specificity is shown, although the usual group reactions 

 occur here as elsewhere in immunology. Their appearance is often more marked in 

 testing sera from animals subjected to a prolonged course of immunization. In test- 

 tube experiments in which large amounts of precipitinogen are included a prezone 

 may be noticed,^ The reaction of the medium and the presence and character of 

 electrolytes markedly influence the precipitin reaction even to the extent of determin- 

 ing in a large measure whether or not a visible reaction will occur. Despite the large 

 amount of work done upon precipitins, their exact manner of action has not been 

 learned. Ordinary precipitins no doubt consist somewhat of mixtures^ of antibodies 

 since most precipitinogens are mixtures. Therefore much of the work on the proper 

 ties of precipitins must of necessity be repeated with simple antigens. 



' Obermeyer, F., and Pick, E.: ibid., 17, 265. 1904; 19, 327. 1906. 



' Cf. chaps. 1-lv, lix, Ixx, Ixxv-lxxviii of this volume. 



2 Opie, E.: /. Immunol., 8, 19. 1923. 



^von Dungern, F.: Centralbl.f. BakterioL, Abt. I, 34, 355. 1903, 



