PRECIPITINS 49 



cells of a foreign species, its serum becomes agglutinative to these 

 cellsp 



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

 nized by repeated injections of blood foreign to it, 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 blood against which the 

 animal was immunized. As soon as the immunized serum and the 

 specific blood are mixed, a precipitate forms. This is another 

 phenomenon of immunity, and is of more than theoretical import- 

 ance 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. 

 Naturalists also use this method for the differentiation of species. 

 By many, 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 union occurs 

 between the toxin and the anti-toxin, since, according to the law of 

 multiples, a definite amount of anti-toxin unites with a definite 

 4 



