PRECIPITINS 229 



dilution of antigen that yields a definite ring at the junction with 

 immune serum. Small round-bottomed fermentation vials may 

 be used in this work. A satisfactory serum should react within 

 two to twenty minutes at room temperature. 



Range of Specificity. — As soon as the titer of an immune 

 serum, e.g., antihuman, has been determined, it is necessary to 

 ascertain the range of its specificity. In blood work this is accom- 

 plished by setting up a series of titrations, using dilutions of 

 various other bloods, as, for example, Hektoen mentions fish, 

 chicken, rabbit, guinea ]>ig, ]"at, cat, dog, swine, sheep, beef, lioi'se, 

 goat, monkey and luunan. In the case of liigh titcrcd antihuman 

 precipitin serum, one will find tliat except for the blood of 

 anthropoid apes, cross reactions either will not occur or will be evi- 

 dent in very low dilutions such as 1 :10. Ape blood may react in 

 dilutions of 1 :100 or 1 :200, but not in high dilutions sucli as 

 1 :1,000, 1 :r),000 or 1 :10,000 which the homologous human blood 

 gives. When one is working with an antibeef serum one finds that 

 sheep l)lood (closely related species) gives reactions in fairly high 

 dilutions ; absorption experiments, however, will differentiate 

 clearly l)etween the two. Otlier closely related species are tlie 

 horse and mule ; dog, wolf and fox ; domestic fowl, turkey, goose, 

 duck and pigeon ; hare and rabbit. 



Practical Use of the Test. — According to Fornet and Midler 

 (1910) and Hektoen (1928) the ring test is satisfactory for all 

 medicolegal work. Dean (1931), however, feels that it is satis- 

 factory for preliminary work but that one should consider the 

 influence of optimal proportions of antigen and antibody upon 

 the reaction in performing the final test. 



Adsorption and Agglutination Technique. — In Chapter XI 

 attention is called to a paper by Cannon (1940) in which he de- 

 scribes a more rational method of titrating precipitins. This 

 method is delicate and sharply specific. It promises to be of value 

 for the quantitative study of the relationship of precipitins to 

 various types of hypersensitive reactions. 



Optimal Proportion of Immune Serum and Antigen. — In order 

 that the student may ))etter appreciate some of the conditions 

 necessary for the formation of immune precipitates preparatory 

 to a discussion of the Ramon floeeulation technique for the titration 



