828 PRECIPITINS AND THEIR APPLICATIONS 



which a simple antigen, highly purified egg albumen, is introduced into an animal 

 with a high precipitin titre against this substance, a severe drop in titre would be an- 

 ticipated if the injected dose were large. The fall in the titre may be to zero, and fresh 

 antigen may appear in the blood. At this time there is no simple precipitin left in the 

 circulation. When it reappears, the precipitinogen disappears. 



It seems that precipitins are involved, at least in some cases, in the mechanism by 

 means of which the immune animal differs from the normal and is enabled to free itself 

 readily of foreign protein. The immune animal does this quickly; the normal animal 

 does it slowly. 



PRODUCTION OF PRECIPITIN 



It is obvious that the precipitin reaction is of value first of all in detecting protein. 

 The sensitivity of the test is such that in some cases protein present in a concentra- 

 tion of one part in one hundred thousand or more can be detected. The superior sensi- 

 tivity of the reaction over chemical tests is apparent. Satisfactory immune-serum 

 titres can usually be obtained in laboratory animals using most unformed proteins 

 for antigen, and giving a course of three to six doses of antigen so spaced as to avoid 

 shocking the animal. The higher the titre of the precipitin serum the more satis- 

 factory it is for practical use, since confusing cross-reactions may be eliminated more 

 easily. In order to obtain large amounts of immune serum, laboratory animals may 

 be given a very short rest after a bleeding, then desensitized with two or three graded 

 subcutaneous doses during a few hours and given a second or even third series of im- 

 munizing injections and subsequent bleedings. Since very clear serum is desirable 

 in precipitin tests, some degree of fasting of the injected animals just previous to 

 bleeding is desirable. Most preservatives have a tendency to cause clouding of serum, 

 especially with older sera. Better results may be obtained by using unpreserved im- 

 mune serum, handling it under precautions to insure sterility, and storing it at 

 temperatures near freezing. 



In preparing a series of tests to detect and identify unknown proteins it is found, 

 in the production of desirable antisera, that not all common proteins are equally anti- 

 genic, and variable antiserum titres are obtained against different substances no mat- 

 ter what the method of treatment of animals furnishing the sera may be. The precip- 

 itin titres vary much with individual animals and also greatly when different species 

 are used for injection. For instance, rabbit anti-beef precipitin of high titre is rather 

 easy to obtain, while rabbit anti-human precipitin is more often of lower titre. Also 

 for the most part rabbits produce more highly potent precipitin than guinea pigs. 

 The blood of each animal should be kept separate because pooled blood sometimes 

 shows clouding and precipitation. 



Bacterial precipitins are not so regularly or so easily obtained as precipitins against 

 unformed, innocuous proteins such as native serum proteins. Generally a longer 

 course of treatment is necessary to incite high bacterial precipitin titres. After the 

 usual injections, the major defensive forces of the treated animal may be involved in 

 producing agglutinin, sensitizer, etc., instead of precipitin, and the latter may appear 

 late. Detoxification of particularly toxic micro-organisms may aid in the preparation 

 of more highly potent bacterial precipitin. 



