44 IMMUNE SERA 



the cell proper. This type of receptor suffices for 

 comparatively small molecules such as those of the 

 toxins, for these are, after all, but the products of 

 cellular activity. When the protoplasm of the 

 bacterial cell itself, however, is to serve as food for 

 the animal cell the latter needs more than a mere 

 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 



