FORMATION OF PRECIPITINS 295 



complements. As with agglutinins, the presence of a salt is necessary 

 to secure the reaction. 



The haptophore or combining arm is quite stable; the precipito- 

 phore group is more labile, and is affected by heat, and when this less 

 resistant arm is lost, the receptor is called a precipitoid. Like agglutin- 

 oids, the precipitoids are of practical interest from the fact that their 

 haptophore arm will not only combine with precipitinogen, but displays 

 a greater activity in this direction than the whole receptor or precipi- 

 tin itself, and when union between precipitinogen and precipitoid has 

 occurred, precipitation does not result. Hence in low dilutions of a 

 precipitin serum the phenomenon of precipitation is slight or altogether 

 absent, whereas in higher dilutions the reaction becomes evident. 



Group precipitins are not so prominent as group agglutinins, yet they 

 are formed to a certain degree and are of much practical importance in 

 attempting to differentiate bacteria and serums by the precipitation 

 method. Although precipitins are highly specific, the principle of serum 

 dilution, as emphasized under Agglutination, must be closely observed 

 in order to dilute the group precipitins to such small amounts as to pre- 

 vent them from interfering with the chief precipitin. This principle is of 

 particular importance in differentiating the bloods of various animals, 

 and especially in medicolegal cases, where the precipitin reactions are 

 employed for the diagnosis of blood-stains. 



Formation of Precipitins. Immune serums for diagnostic purposes 

 are produced by injecting the precipitinogenous fluid into the veins, 

 peritoneal cavity, or subcutaneous tissues of animals, usually rabbits. 

 The power of forming precipitins is probably disseminated among the 

 organs and general body tissues. Kraus and Levaditi assign the leu- 

 kocytes as the chief source of precipitin formation. 



As in the case of agglutinin formation, not all animals possess equally 

 the power of forming a precipitin for a given albumin. While this point 

 is of general interest with the bacterioprecipitins, it becomes of particular 

 importance in relation to serum precipitins. For example, an animal 

 will not form a precipitin active against its own serum. If formed, it 

 would be an autoprecipitin, or isoprecipitin, and, as a rule, animals do 

 not form antibodies for their own tissue constituents. Furthermore, 

 animals are unlikely to form precipitins for the proteins of other members 

 of the same species, or if precipitins are produced, they are usually the 

 result of prolonged immunization of a number of animals. Precipitin 

 formation is also slight for the proteins of other animals that are closely 

 related either zoologically or biologically. For example, attempts at 



