THE DEFENSES OF THE BODY 289 



blood of animals artificially immunized from cholera, pyo- 

 cyaneus, typhoid, dysentery, and colon infections also show 

 the presence of "agglutinin." So far as experience has gone, 

 this agglutinating property is manifested in the great major- 

 ity of cases only upon the particular organisms from which 

 the animal supplying the serum is protected; that is to say, 

 the relation is specific. In view of the fact that the power 

 of a serum to agglutinate bacteria is regarded by many as a 

 concomitant of infection, the exhibition of this property by 

 the blood of immune animals may at first sight appear 

 paradoxical. We should not lose sight of the fact, however, 

 that agglutinin is presumably distinct from the other sub- 

 stance concerned in immunity, and its presence in immune 

 animals may, therefore, be reasonably explained as a more or 

 less permanent result of the "reactions of infection" that were 

 coincident with the primary stimulations by specific infec- 

 tive or intoxicating matters necessary to the establishment 

 of the condition of immunity; nor should we in this con- 

 nection lose sight of the fact that its presence is constantly 

 to be demonstrated in typical cases of typhoid fever, for 

 instance, that terminate fatally, and that have exhibited 

 little or no clinical signs of resistance at any time during 

 their course. 



Fifth, there may be demonstrated in the blood of animals 

 that have received repeated subcutaneous injections of 

 milk a body — a "precipitin" — that causes a precipitation 

 of milk. This precipitation represents apparently a specific 

 reaction, for it occurs only when the blood-serum is mixed 

 with milk from the species of animal that supplied the milk 

 used for immunization. 



Sixth, after the repeated injection of blood or of emulsions 

 of tissue-cells into the body of an animal, there appear in 

 19 



