586 IMMUNITY 



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 re- 

 garded as composed of a central atom group or functional centre 

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

 ketten), i.e., atom groups with combining affinity for food- 

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

 cule 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. 558) ; 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 

 anti-substance enters into combination with its corresponding 

 substance antigen. The dual constitution of toxins and kindred 

 substances, as already described (p. 203), 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 toxophorous part of the toxin 

 molecule produces that disturbance of the protoplasm which 

 is shown by symptoms of poisoning. If, however, they are 

 in smaller dose, as in the early stages of immunisation, fixation 

 to the protoplasm occurs in the same way j and as the com- 

 bination of receptors with toxin is supposed to be of firm 

 nature, the receptors are lost for the purposes of the cell, and 

 the combination R.-T. (receptor + toxin) is shed off into the 



