7^0 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



the precipitoids of the precipitinogen in distinction from the predpitoids 

 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 Uttle 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 

 to determine whether a particular stain was blood or not. When it 

 came to determining the exact species from which the blood came it 

 was impossible. By means of the precipitins this can be done. For 

 example, a stain which is supposed to be blood is carefully dissolved 

 out in 0.85 per cent sodium chloride solution and placed in a sterile 

 test-tube. A series of animals, such as rabbits, have been immunized 

 to the various known blood sera and after immunization their sera are 

 drawn off. These sera contain the precipitins for the various sera and 

 corpuscles used in immunization. These precipitins are combined 



