DISEASE, INFECTION, AND RESISTANCE 293 



injected into the peritoneal cavity of a suitable animal, 

 samples withdrawn at intervals and stained preparations 

 made for microscopic observation, the bacteria were found 

 at first to stain sharply and definitely, later to stain 

 indefinitely, then as granules, and at last completely to 

 disappear. It was then found possible to immunize animals 

 against various disease-producing organisms and to study in 

 the blood serum of such immunized animals the substances 

 capable of destroying the organism. Such substances were 

 named bacteriolysins. Later it was found that the injection 

 of other kinds of foreign cells would lead to the develop- 

 ment of corresponding antibodies. For example, if red 

 blood corpuscles (carefully washed to remove serum) of a 

 guinea pig be injected into the blood of a rabbit, the latter 

 will finally acquire the property of dissolving or digesting 

 the red blood cells of the guinea pig. This phenomenon may 

 be observed in a test tube. If a suspension of guinea pig 

 corpuscles in physiological salt solution be made and a drop 

 of blood serum from a rabbit which has been immunized 

 against such corpuscles be added to it, the opaque red sus- 

 pension will change in the course of a few minutes or half 

 an hour to a transparent, red solution. Such an antibody 

 capable of bringing about solution of red blood cells is 

 termed an hemolysin. Other types of cells were also found 

 to be capable of inciting the development of lytic substances. 

 Antibodies of this general type capable of dissolving cells 

 are termed cytolysins. 



Constitution of Cytolysins. Cytolysins are thermola- 

 bile, that is, when exposed to a temperature of 56 c. or 

 above for a half hour, they lose their power to destroy 

 cells. However, this power to destroy cells may be com- 

 pletely restored by adding to the antiserum some serum 

 from normal blood, that is, serum from an animal which 

 has never been immunized. It is evident, therefore, that a 

 cytohjsin is in reality made up of two substances; one pro- 



