EHRLICH'S SIDE-CHAIN THEORY 549 



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 

 pro forties 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 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 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. 521); 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. 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 acquired 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 

 ;mti -substance enters into combination with its corresponding 

 substance antigen. The dual constitution of toxins and kindred 

 substances, as already described (p. 198), is also of importance in 

 this connection. Now, to take the case of toxins, when these 

 aiv introduced into the system they are fixed, like fund stiitt's, 

 1>\ tlirir h;i|>ti>]iliorous groups to the receptors of the cell 

 protoplasm, but are unsuitable for assimilation. If they are in 

 sutliciently large amount, the toxophorous part of the toxin 

 molecule produces that disturbance of the protoplasm which 



