490 IMMUNITY. 



to produce substances which are inimical to their growth, so that 

 they die out, just as they do in a test-tube culture before the 

 medium is really exhausted. Such a theory only survives now 

 in the view that antitoxins are modified toxins, the evidence 

 against which has already been discussed (p. 475). There then 

 came the humoral theory and the tJieory 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 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 prop- 

 erties 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. 



i. Ehrlich's Side-chain Theory. This may be said to be an 

 application of his views regarding the nourishment of protoplasm. 

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

 as composed of a central atom-group (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 two chief kinds : the first 

 has a single unsatisfied combining group and fixes molecules of 

 simpler constitution receptor of the first order ; the second has 

 two such groups, one for the food molecule and another which 

 fixes a ferment in the fluid medium around receptor of the 

 second 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 



