54 IMMUNITY 



it. Sera can be rendered so highly agglutinative as to produce 

 this reaction even if diluted 100,000 times or more. 



If an animal is immunized against spermatozoa, or the red 

 blood cells of a foreign species, its serum becomes agglutinative 

 to these cells. 



Precipitins. If a rabbit, or any other animal in fact, is immun- 

 ized by repeated injections of foreign protein (blood, bacterial 

 culture, etc.) peculiar bodies develop in its blood serum called 

 precipitins, and these can be demonstrated by adding to the 

 serum of the immunized animal in a test-tube a minute portion 

 of the material against which the animal was immunized. As 

 soon as the immunized serum and the specific substance are 

 mixed, a precipitate forms. This is another phenomenon of 

 immunity, and is of more than theoretical importance in medicine. 

 The reaction is strictly specific; thus, if the serum of a goat is 

 injected into a rabbit repeatedly the rabbit's blood will form a 

 precipitate with normal goat's serum if the two are mixed in 

 a test-tube. Old dried blood, semi-putrid blood, blood on white- 

 wash, or rusty steel, even in minute quantities, if dissolved in salt 

 solution, may be used to produce this reaction. In medico-legal 

 matters, this test is of use for the identification of human blood. 

 By some, the phenomenon of agglutination is supposed to be due 

 to the formation of a precipitin, in the meshes of which bacteria or 

 blood cells are caught and agglutinated, and that agglutination is 

 but a modification of the formation of precipitins. 



Anti-toxin formation is also another phenomenon of immunity. 

 If an animal, such as a horse, receives numerous increasing doses 

 of a given toxin, say that of tetanus, it, in a short time, becomes so 

 accustomed to the poison, that it can withstand the administration 

 of immense doses. (If these large doses had been given at first, 

 they would have proved fatal.) If the horse is then bled, and its 

 serum injected into rabbits or guinea pigs, they may receive 

 shortly after, at one dose, enough toxin to kill ten such animals. 

 The horse serum thus protected these animals against the toxin, 

 as it was anti-dotal, or in other words anti-toxic. A chemical 



