IMMUNITY AND SUSCEPTIBILITY. 573 



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

 of the trace of organic acids materially facilitate this reaction. Further- 

 more, 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 "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 immu- 

 nized, 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 'mmunized 

 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 substances specific to this phenomenon in common in the 

 blood serum of the monkey and man. There are other rare instances 

 of coprecipitins which will not be discussed 



The Forensic Use of Precipitins. The precipitins on account of their 

 great specificity are of use in the identification of various albuminous 

 substances. They have been used, for example, in the identification of 

 bloods. Before the knowledge of the precipitins was at hand, the only 

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

 microscope in examining the corpuscles. If the corpuscles were in a 

 good condition, it was possible to differentiate between a mammalian and 

 fowl blood, on account of the nucleation of the corpuscles of the latter. 

 By the use of the spectroscope it was possible to determining whether a 

 particular stain was blood or not. When it came to determine the exact 

 species from which the blood came it was impossible. By means of the 

 precipitins this can be done. For example, taken a stain which is sup- 

 posed to be blood, this is carefully dissolved out in 0.85 per cent salt 

 solution and placed in a sterile 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 separately in small test-tubes with the 

 salt solution preparation of the blood in question. A precipitate occurs 

 when the corresponding precipitating serum is added. It is necessary, 

 of course, to place these preparations in the incubator at 37. By this 

 method all types of mammalian blood may be separated from each 

 other with the possible exception, as before stated, of monkey and 



