152 A MANUAL OF BACTERIOLOGY 



some it has been supposed that the antitoxin is modified 

 toxin, the modification being brought about by the vital 

 activities of the cells. But the amount of antitoxin 

 produced does not necessarily bear any relation to the 

 quantity of toxin injected. Woodhead records instances 

 in which the amount of antitoxin formed amounted to 

 40,000 times the equivalent amount of toxin injected, 

 bleeding the animal only temporarily reduces the anti- 

 toxin content of the serum, and substances which increase 

 the secretive properties of glandular cells, such as pilo- 

 carpine, enormously increase the output, so to speak, of 

 antitoxin. 



In view of these facts Ehrlich elaborated his " side- 

 chain theory," a theory which, whether it be the real 

 explanation or no, has received a considerable amount of 

 experimental support, and has had far-reaching effects 

 in stimulating research. Ehrlich believes that the chemical 

 activities which are the manifestations of the vital 

 activities of the living cell are due to a very large nucleus 

 or chemical molecule having a ring structure, analogous 

 to the benzene ring, and having attached to it a number 

 of atomic groups or " side- chains." A " side- chain " is 

 an atomic group, a carbon atom of which is linked to one 

 of the carbon atoms in a ring. These atomic groups or 

 side-chains are unstable in nature, and enter freely into 

 combination with other suitable groups should these be 

 presented to them, and thus the physiological activities 

 of the cell, assimilation, nutrition, etc., are carried out 

 (Fig. 26). Now Ehrlich supposes that antitoxin is merely 

 an excess of certain side-chains which are normally present 

 and subserve some of the ordinary functions of the cell 

 and which have become free in the blood. The antitoxins 

 being specific, by this assumption the difficulty is obviated 

 of supposing that special chemical groups or molecules 

 exist preformed ready to combine with a number of 



