72O MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



have in immune reactions of such a substance. The antiantibody is 

 the limit for antibody formation. 



The Precipitinogen. As before stated, the precipitinogen is any 

 protein substance which will cause the formation of precipitins. Cer- 

 tain of the precipitinogens are composed of two groups, one which is 

 thermostable and another which is thermolabile. Therefore, when 

 these precipitinogens are heated and this thermolabile substance de- 

 stroyed there results a substance which is exactly similar to the precipi- 

 toid produced by heating the precipitin. Such bodies are known as 

 the precipitoids of the precipitinogen in distinction from the precipitoids 

 of the precipitin. These precipitoids retain their power to combine with 

 precipitin, but no precipitate results on such combination. 



The Precipitate. When precipitin and precipitinogen combine it 

 requires some little time before precipitation occurs. This is dependent 

 upon the temperature (37 best) and certain other factors. The 

 presence of the trace of organic acids materially facilitates this reaction. 

 Furthermore, the reaction will not take place without the presence of 

 certain electrolytes or salts. 



Coprecipitins. The phenomena of " group precipitation" does not 

 occur as often as does " group agglutination." The bacterial precipitins 

 are very markedly specific but some of the blood precipitins are not 

 so specific. For example, in a case where two rabbits have been im- 

 munized, one with the blood serum of man and the other with the blood 

 serum of the monkey, it is found that the serum of the rabbit immunized 

 to human blood serum will precipitate monkey blood serum to a less 

 degree, of course, than human serum. This is due to the fact that there 

 are certain chemical substances in common in the blood sera of the 

 monkey and man. There are other rare instances of coprecipitins 

 which will not be discussed. 



The Forensic Use of Precipitins. The precipitins are of use on ac- 

 count of their great specificity in the identification of various albumi- 

 nous substances. They have been used, for example, in the identifica- 

 tion of bloods. Before the knowledge of the precipitins was available, 

 the only means of determining one blood from another was by means of 

 the microscopic examination of the corpuscles. If the corpuscles were 

 in a good condition, it was possible, for example, to differentiate between 

 a mammalian and fowl blood, on account of the nucleation of the cor- 

 puscles of the latter. By the use of the spectroscope it was also possible 



