THE DEFENSES OF THE BODY 283 



Precipitins. The immunization of animals with a variety 

 of substances other than bacteria has served further to de- 

 monstrate the complex mechanism of immunity. One of the 

 reactions that is noticed as the result of such immunization 

 is the precipitation observed when the serum of the immu- 

 nized animal is mixed with the substance with which it has 

 been treated. For instance, the serum of an animal that 

 has received repeated injections of blood, tissue juices or 

 certain secretions from alien species, will cause a precipi- 

 tate to form when mixed with either of these substances 

 in vitro. These "precipitins," as the newly formed bodies 

 in the blood of the treated animal are called, are specific 

 in that they form precipitates only with the materials 

 injected. 



This precipitin reaction is so characteristic that it is 

 employed for the identification of blood in medico-legal 

 cases requiring the differentiation between human blood 

 and that of domestic animals; thus, the serum of a rabbit 

 into which human blood has been injected will cause a 

 precipitate with no other blood except that of the anthro- 

 poid ape. 



In like manner, the repeated injection of milk of one 

 species of animal into the tissues of another will result in 

 the formation of specific precipitins in the blood serum of 

 the treated animal, that will precipitate only the milk of 

 that species of animal from which the milk was derived. 



Agglutinins. If the blood serum of an individual who has 

 recovered from a bacterial infection or who has been ren- 

 dered immune by bacterial vaccination be mixed with the 

 bacteria that caused the infection or those used in the vac- 

 cination the bacteria, if motile, lose their motility and 

 finally clump together in masses, i. e., they are "agglu- 



