INFECTION AND IMMUNITY 363 



noted after the injection: the animal appears well, con- 

 tinues well, and shows no sign of having experienced any 

 disturbance. It has, however, undergone a profound 

 constitutional, change in which the physiologico-chemical 

 balance has been completely upset, for if it be now given 

 a second injection of only 0.1 to 0.2 c.c. of the same 

 serum, it within a few moments becomes greatly dis- 

 tressed, seems to suffer from embarrassed circulation, 

 violently scratches the face and nose, gasps for breath, 

 falls upon its side more or less convulsed, and may die 

 within an hour. This condition is ascribed to a hyper- 

 sensitivity to the horse serum effected by the first or sensi- 

 tizing dose, and is described as allergia or anaphylaxis. 

 Thus the single large dose is not followed by visible effects; 

 frequent small doses, coming one after another too 

 frequently to permit anaphylaxis, result in immunity to 

 the ill effects; but the second dose, properly spaced 

 after the sensitizing dose has effected its disturbance, 

 is fatal. The nature of the anaphylactic reaction is 

 not understood, but the disturbances resulting from it 

 are profound and are accompanied by histological altera- 

 tions affecting many of the tissues of the animal and 

 abundantly explaining its death. 



Reactions of this kind are effected by many het- 

 erologous protein substances, though they are rarely 

 so profound or so serious as in the case chosen for 

 illustration. 



The quality of the antigen determines the nature 

 of the antibody formed by the reaction following its 

 administration, and in order that the full force of the 

 antigen may be effected it is usually necessary that it be 

 admitted to the blood directly by absorption from the 

 subcutaneous tissue or from one of the serous cavities 

 rather than by introduction into the alimentary tract 

 where it is apt to be transformed and rendered inert. 



The reader must not jump to the hasty and erroneous 

 conclusion that any heterologous substance may serve 

 as an antigen, and produce antibody formation. Only 



