250 EESISTANCE AND IMMUNITY 



In methyl salicylate, for example, the configuration is changed in this 

 way: 



OH 



CO2CH3 



In a corresponding manner, it has been suggested by EhrHch that 

 the toxins are capable of exerting their destructive action upon the 

 body by becoming chemically bound to the cells through the inter- 

 vention of the receptors of the latter. These are specific in their 

 action and cease their function as soon as they have combined with 

 the toxic substance. They are eventually broken down and de- 

 stroyed. Thus, when the haptophore group of the toxin has been 

 anchored to a receptoric side-chain, this particular feeler of the cell 

 is of no further use and is cast off. The formation of the antitoxic 

 substance depends upon the physiological reaction of the cell to this 

 injury. In all probability the latter will endeavor to compensate 

 for the loss of its receptors by the formation of new ones; indeed, the 

 constant stimulation by the toxin will finally lead to an over-produc- 

 tion of receptoric substance which, after its disconnection from the 

 cell, is transformed into free circulating antitoxic substances. The 

 latter represent the so-called antibodies which exert their protective 

 action even in regions of the body far removed from their place of 

 origin. 



The side-chain theory of Ehrlich, therefore, furnishes a means for 

 explaining the union between the cells of the animal and the toxins, as 

 well as the formation of the antibodies. In the form just given it 

 fails, however, to account for several phenomena frequently observed 

 during experiments on immunity; for example, it has been noted that 

 certain pathogenic bacteria against which the body is resistant, do not 

 stimulate the formation of antibodies. Moreover, it has been ascer- 

 tained a long time ago that the normal function of blood-serum to 

 destroy certain pathogenic bacteria, may be wholly removed by sub- 

 jecting it to a temperature of 56° C, but a serum which has been 

 rendered inactive in this way, may be reactivated by the addition of 

 a small quantity of normal serima. 



These and other facts have led to the assumption that the immune 

 substances appear in the form of two, namely, as a thermolabile body, 

 known as "alexin," and as a more stable body, called ^'sensitizing 

 substance." Thus, it is beHeved that the bacteriolytic action of serum 

 depends upon the combined action of these two substances. The 

 first plays the part of the principal agent, and the second, that of the 

 binder which renders the bacteria vulnerable. In the terminology 



