186 A MANUAL OF BACTERIOLOGY 



tophore groups remain unaffected. A toxin molecule 

 with intact toxophore group seems, however, to be more 

 stimulating for the production of antitoxin than a toxoid 

 in which this group is absent. For the production of 

 antitoxin, therefore, active toxin is always employed. 



It is the presence of the hap tophore group which con- 

 ditions the union oi toxin with antitoxin. Thus, if toxin 

 be injected into the blood containing antitoxin, the hap- 

 tophore groups of the toxin unite with the free receptor 

 groups, i.e. with the antitoxin (Fig. 32), and therefore the 

 toxophore groups cannot exert their influence because the 



toxin is now unable to unite 

 with the protoplasm, its hap- 

 tophore or binding groups 

 being already occupied. 



In a poisonous toxin, such 

 as diphtheria or tetanus toxin, 

 the toxophore group is more 

 readily destroyed than the 

 haptophore group. A toxin 

 FIG. 32. Neutralisation of heated to 65-70 C. or kept 



toxin by antitoxin in the f O r some time loses toxicity 

 blood. (After Ehrlich.) but &m ^^ an affinity 



for antitoxin. If antitoxin be treated with such heated 

 or old toxin and its capacity for neutralising active 

 toxin be then tested, this will be found to be diminished 

 because, although toxophore groups are absent from the 

 old or heated toxin, the binding or haptophore groups 

 still remain and saturate some of the antitoxin. Such 

 derivatives of toxin possessing haptophore, but not 

 toxophore, groups are termed " toxoids." Wassermann 

 and Bruck have obtained presumptive evidence of the 

 existence of the second stage in antitoxin formation, viz. 

 the increased production of receptors by the cells. Using 

 old tetanus toxin which had lost its toxicity, but which 



