EHRLICH'S SIDE-CHAIN THEORY 491 



be in part the source of important bodies in the serum. At the 

 present time interest centres around two theories, viz. Ehrlich's 

 side-chain theory and Metchnikoff' s 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, viz. that of nutrition. 



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

 application of his views regarding the nourishment of proto- 

 plasm. A molecule of protoplasm (in the general sense) may be 

 regarded as composed of a central atom group or executive 

 centre (Leistungskern) with a large number of side -chains 

 (Seitenketten), i.e. atom groups with 'combining affinity for 

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

 molecule is increased in tfre 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. 466) ; the first has a 

 single unsatisfied combining group and fixes molecules of 

 simpler 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 com- 

 bining groups, one for the food molecule and another which 

 fixes a ferment in the fluid medium around receptor of the 

 third order or amboceptor. These latter 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 passive immunity, it must be kept in view that all the 

 substances to which anti-substances have been obtained are, like 

 proteids, of unknown but undoubtedly of very complex chemical 

 constitution, and that in apparently every case the anti-substance 

 enters into combination with its corresponding substance. The 

 dual constitution of toxins and kindred substances, as already 

 described (p. 170), is also of importance in this connection. 

 Now, to take the case of toxins, when these are introduced into 

 the system they are fixed, like food-stuffs, by their haptophorous 

 groups to the receptors of the cell protoplasm, but are unsuitable 

 for assimilation. If they are in sufficiently large amount the 



