374 DEFENCE 



When a toxin is heated to 60° it loses its toxic power 

 but still retains the capacity to form an antitoxin when 

 injected. Thus modified it is known as a toxoid. It is 

 therefore beheved that a toxin contains two molecular 

 groups, one the haptophore which unites with the receptor 

 of the protoplasm, the other the toxophore group which 

 can only exert its action when the haptophore group is 

 hnked to the cell. In the toxoid the toxophore group only 



Fig. 71. 



is destroyed, the haptophore group being still capable of 

 stimulating the production and hberation of receptors 

 (Fig. 72). 



From the fact that the capacity to induce the formation 

 of neutrahsing substances is found in such widely differing 

 substances as benzoic acid and bacterial toxins, it is not 

 surprising to find this property widely possessed by many 

 other classes of substances. Any substance which has this 

 property is called an antigen, and the substance produced 

 in the body, an antibody. Any foreign protein, for instance, 



