INFECTION AND IMMUNITY. 611 



nor should we in this connection lose sight of the fact 

 that its presence is constantly to be demonstrated in 

 typical cases of typhoid fever, for instance, that termi- 

 nate 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 ani- 

 mals that have received repeated subcutaneous injections 

 of milk a body a " precipitin " that causes a precipi- \/ 

 tation 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 the injections. Sixth, 

 after the repeated injection of blood or of emulsions of 

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

 the blood of that animal certain solvents, or enzyme-like 

 bodies, " haemolysins," " cytolysins," etc., that react 

 specifically upon the blood or tissue-cells injected ; 

 agglutinating, disintegrating, and finally completely 

 dissolving them. Here, too, the relations are specific. 

 If a rabbit, for instance, be rendered tolerant to or 

 immune from beef-blood, its serum dissolves only the 

 red corpuscles of bovines ; if from dog's blood, then 

 only the corpuscles of the dog are dissolved by the 

 serum of the rabbit. Similarly, if a rabbit be rendered 

 tolerant to injections of emulsions of epithelium cells, 

 then its serum dissolves epithelium and not other cells. 

 All these reactions may be seen in a test-tube or under 

 the microscope. Seventh, if a hsemolyzing serum, pre- 

 pared as indicated under the sixth observation, be heated 

 for a short time to 54-56 C., it at once loses the 

 hsemolytic function, but regains it again if a few drops 

 of serum from a normal animal be added to it. In this 

 phenomenon of haemolysis Ehrlich's " receptors of the 



