EHELICH'S SIDE-CHAIN THEORY 585 



against which has already been discussed (p. 568). There then 

 came the humoral theory and the theory of phagocytosis, but 

 neither of these is tenable in its pure form, and the distinction 

 between them need not be maintained. For, on the one hand, 

 any substance with specific property in the serum must be the 

 product of cellular activity, and, on the other hand, the facts 

 with regard to passive immunity go far beyond the ingestive and 

 digestive properties of phagocytes, though these cells may be in 

 part the source of important bodies in the serum. At the pre- 

 sent time interest centres around two theories, namely, Ehrlich's 

 side-chain theory and Metchnikoffs phagocytic theory as further 

 developed. These will now be discussed, and it may be noted 

 that the ground covered by each is not coextensive. For the 

 former deals chiefly with the production of anti-substances and 

 its biological significance, the latter deals with the defensive 

 properties of cells, either directly by their phagocytic activity 

 or indirectly by substances produced by them after the manner 

 of digestive ferments. It will be seen, however, that each has 

 a normal process as its basis, namely, that of nutrition. 



1. Ehrlich's Side-Chain Theory. — This may be said to be an 

 application of his views regarding the nourishment of cells. A 

 molecule of protoplasm (in the general sense) may be regarded 

 as composed of a central atom group or functional -centre with a 

 large number of side-chains, i.e., atom groups with combining 

 affinity for food-stuffs. It is by means of these latter that the 

 living molecule is increased in the process of nutrition, and hence 

 the name receptors given by Ehrlich is on the whole preferable. 

 These receptors are of three chief kinds corresponding to the 

 classes of anti-substances described (p. 561); the first has a 

 single unsatisfied combining group, and merely fixes molecules 

 of relatively simple constitution — receptor of the first order ; 

 the second has a combining group for the food molecule, and 

 another active or zymotoxic group, which leads to some physical 

 change in it — receptor of the second order ; the third has two 

 combining groups, one for the food molecule and another which 

 fixes a ferment (or complement) in the fluid medium around — 

 receptor of the third order or amboceptor. The last receptors 

 come into action in the case of larger food molecules which 

 require to be broken up by ferment-action for the purposes 

 of the cell economy. In considering the application of this idea 

 to the facts of acquired immunity, it must be kept in view that 

 all the substances to which anti-substances have been obtained 

 are, like proteins, of unknown but undoubtedly of very complex 

 chemical constitution, and that in apparently every case the 



