1158 



PHYSIOLOGY 



enables us to classify the very complex phenomena of immunity in a 

 more or less imperfect fashion. 



The property of giving rise to anti-bodies on injection into an animal 

 is not confined to toxins, a large number of substances, e.g. egg albumin, 

 serum, proteins, ferments, albumoses, partaking of the same property. 

 All such substances are classed together as antigens. Thus human 

 serum injected into a rabbit produces in the rabbit's serum some 

 body which will give a precipitate when mixed with human serum 



FIG. 484. Schematic representation of formation of antitoxin as side-chains of 

 protoplasmic molecule. The black bodies are the toxin molecules which fit 

 by their haptophore end on to the side-chains of the cell. (EHRLICH.) 



even in minute traces. This precipitin formation is specific, so that 

 it may be used as a test for the origin of any unknown specimen of 

 serum. In the same way rennet ferment when injected gives rise to the 

 production of an anti-rennin which will neutralise the action of this 

 ferment on milk. Antigens are all colloidal in character and probably 

 optical y active. Ordinary drugs do not give rise to the formation of 

 anti-bodies, a necessary condition being apparently some similarity in 

 the molecular structure of the antigen to the protoplasm of the animal 



