64 IMMUNE SERA 



Third Order. Ehrlich's theory as outlined in the 

 preceding chapter offers a ready explanation for the 

 development of these bodies. Certain peculiarities 

 of the agglutinins require merely a slight elabora- 

 tion of detail in order to be clearly understood. 

 According to Ehrlich the prime function of the side 

 chains of a cell is to provide for the nutrition of the 

 cell. Obviously the simplest mechanism for this 

 purpose will be a side chain which merely anchors 

 the food molecule, leaving the digestion entirely to 

 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 



