154 IMMUNITY. THEORIES OF IMMUNITY 



none is in a position to cause the production of antitoxins, for alkaloids 

 possess no haptophore group that anchors them to the cells of organs. 

 As has been stated, in the formation of antitoxin the haptophore 

 group of the toxin molecule is the essential portion; the toxophore 

 group is much less important, and during immunization the symptoms 

 of illness due to the action of the latter group are not essential to and 

 play no part in the production of antitoxin. It must be said, however, 

 that a toxin molecule with an intact toxophore group is more stimulating 

 than a toxoid in which this group is absent; therefore, in artificially 

 immunizing horses for the production of antitoxin, after the first few 

 injections increasing amounts of toxin are administered. 



Antibodies of the Second Order (Agglutinins and Precipitins). 

 As new discoveries were made, Ehrlich amplified his theory of the for- 

 mation of antibodies, but always upon the original and basic conceptions 

 as just set forth. 



We have seen that the simplest molecules of food substances, toxins, 

 and ferments, substances really in solution, are anchored to molecules 

 of cell protoplasm by means of the simple side-arms of the latter. When 

 this chemical union has taken place, the food or toxin may be assimi- 

 lated without undergoing any further change. With more complex 

 food substances, however, some preparatory treatment is necessary be- 

 fore they become available for final assimilation. The large molecule 

 may readily enough be anchored to the molecule of the cell, but it 

 probably requires some preparation before it becomes available for the 

 nutrition of the cell. 



Accordingly, Ehrlich assumed that the body-cells are furnished with 

 another order of side-chains or receptors composed of two portions; 

 one part or group for union with the food substance, and called the hapto- 

 phore group; and the second portion, called the toxophore or zymophore 

 group, in which the special function of the receptor resides. 



Similarly, certain pathogenic agents that are more complex than 

 soluble toxins or ferments combine with receptors of this kind. One 

 arm, the haptophore group of the receptor, combines with the hapto- 

 phore portion of the pathogenic molecule, and then the second or toxo- 

 phore portion of the receptor exerts some special action upon the at- 

 tached molecule. Receptors or haptines of this nature are known as 

 receptors of the second order; antibodies of the same structure, pro- 

 duced and cast off into the blood-stream as the result of toxic injury 

 and stimulation of body-cells, are known as antibodies of the second 

 order. 



