4 g IMMUNE SERA 



anchoring group, it needs also an active group 

 which can in some way act on the huge food par- 

 ticle and make it more readily assimilable. Such 

 receptors then possess two groups, a haptophore 

 group and another functional group acting on the 

 food particle thus anchored. Ehrlich calls these 

 his " receptors of the second order," and places in 

 this class the agglutinins and the precipitins. The 

 same action can perhaps be more economically 

 brought about by having these receptors, in addi- 

 tion to their specific haptophore group, possess the 

 means by which the action of a ferment-like sub- 

 stance can be brought to bear on the anchored 

 food particle. Such a receptor would then possess 

 two haptophore groups, one for the food particle, 

 the other for the ferment-like substance. These 

 are Ehrlich's " receptors of the third order " and 

 will be discussed in the next chapter. Confining 

 ourselves for the present to the agglutinins we find 

 that the existence of the two groups (haptophore 

 and agglutinating) has experimental confirma- 

 tion. We have seen that an agglutinin may be 

 changed by the action, for instance, of acids, so that 

 it will no longer possess any agglutinating action, 

 but will still combine with the bacteria. Once the 

 agglutinating power is lost it cannot be restored, 

 in which respect the agglutinins differ from the 

 bacteriolysins. 



