242 INFECTION AND IMMUNITY. 



until the reaction is complete, and the sediment is 

 then thrown into the capillary portion of the tube 

 by centrifugation for a stated period of time 

 (twenty minutes). The volume of the sediment 

 may be read by the scale. Nuttall allows the sedi- 

 mentation to occur naturally, with the tubes in an 

 upright position. Other serums naturally must 

 be used as controls. If the "unknown" blood is 

 suspected of being human, a control tube must be 

 prepared in which a similar amount of known hu- 

 man serum is submitted to the same test. If the 

 two tubes yield similar amounts of precipitate 

 when they are treated with 0.2 c.c. of a precipi- 

 tin which is specific for human serum, the identity 

 of the "unknown" blood as that of man is estab- 

 lished. To obtain the specific precipitin it is cus- 

 tomary to immunize rabbits with human serum for 

 several weeks. 



identification Another practical feature of the precipitation 

 test has to do with the differentiation of meats. A 

 precipitogenous substance which is characteristic 

 for the animal may be extracted or pressed from 

 the flesh, and will yield a precipitate when it is 

 mixed with a precipitin of homologous nature. 

 This is of particular interest in those countries in 

 which the meat of the horse is put on the market 

 as a substitute for that of beef. 

 coiioida and (For the relation of precipitins to anaphylaxis 



the Reactions , , i i \ 



of immunity, see chapter on anaphylaxis. ) 



In view of the fact that the protoplasm of the 

 body and the albuminous constituents of serum 

 have a close relationship to, or really are, colloids, 

 investigators have studied certain reactions which 

 occur among the known colloids with the expecta- 

 tion that the reactions of protoplasm and those of 



