No. 637] 



IMMUNE SERA 



In any event tins whole field of endocrinology and 

 serology stands as a perpetual challenge to the experi- 

 mental biologist. Some sixteen years ago Nuttall pub- 

 lished his remarkable series of studies on "Blood Im- 

 munity and Blood Eelationship" in which he reported 

 the results of his examination of some nine hundred dif- 

 ferent samples of blood from various kinds of animals. 

 He demonstrated that by the precipitin test a differential 

 scale of actual blood relationships among animals can be 

 established. As you doubtless recall, when an animal of 

 one species is injected parenterally with successive doses 

 of blood-serum of another species over a period of a few 

 weeks, the blood-serum of the injected animal acquires 

 the ability to form a precipitate with that of the alien 

 species when the two sera are mixed. When the reaction 

 is carried on in vitro, even in dilute solutions, the cloudi- 

 ness and ultimate flocculation which results are easily 

 seen. If, for example^ a rabbit is thus repeatedly in- 

 jected with human blood its blood-serum when mixed 

 with slightly diluted human blood-serum in vitro will 

 almost instantly yield a noticeable precipitate, though a 

 control mixture of human blood-serum and the blood- 

 serum of an untreated rabbit will remain clear. The in- 

 gredient which has been engendered in the serum of the 

 rabbit is termed a precipitin, and the forciun serum which 

 was injected— human blood-serum in tliis case— is called 

 the ant if/en, or more specifically, the prccipitinor/en. It 

 is known that not only blood-serum, but also milk, glob- 

 ulins, various albumins and bacterial products— in fact 

 probably any foreign protein— may serve as antigen for 

 the formation of precipitins. The reaction is not abso- 

 lutely specific in low dilutions since species of animals 

 related to the one from which the antigen was taken will 

 also, though in less degree, give the effect. Closeness of 

 relationship is determined by finding the dilution in which 

 the serum to be tested will react. For instance, Nuttall 

 found that when rabbit serum which has been sensitized 

 against human serum is mixed with the moderately di- 



