EHRLICH'S SIDE-CHAIN THEORY 25 



phore group, which binds it to the cell receptor, and a 

 toxophore group, which actually bears its toxic prop- 

 erties. When the receptor combines with the toxin 

 molecule, the cell throws it off into the circulation, and 

 similar receptors are formed to take its place. These, 

 however, are formed in excess, and the cell throws 

 them off also. These free receptors then imite with 

 corresponding toxin molecules in the circulation. 

 There is a receptor for each particular form of toxin 

 molecule. Thus the diphtheria toxin combines only 

 with the specific receptor provided for it, and will not 

 unite with those intended for tetanus toxin. Ehrlich's 

 theory, which at first only covered the simple union of 

 toxin with antitoxin, was extended also to explain the 

 action of more complex antibodies, and may be said to 

 cover completely all forms of antigen-antibody ac- 

 tion. There are three recognized types or orders of 

 receptors, which are conveniently illustrated by the 

 accompanying diagram (Fig. 1). Receptors of the 

 first order possess only a single haptophore or com- 

 bining group, by which they unite with the hapto- 

 phore group of the corresponding antigens. To this 

 order belong antitoxins and antiferments. Receptors 

 of the second order, in addition to the haptophore 

 group, possess a second or zymophore group, by 

 means of which the anchored antigen can be subjected 

 to further change. Under this head belong agglu- 



