254 Immunity in Some of its Biochemical Aspccts [Jan. 



Rcceptors. In view of what has been said it is obvious that 

 the simplest mechanism by which the cell can lay hold on food par- 

 ticles is a receptor which merely anchors food, leaving the digestion 

 entirely to the cell proper. It may be assumed that this type of re- 

 ceptor suffices for comparatively small food molecules. When a 

 larger and more complex food molecule presents itself, it may be as- 

 sumed that a receptor would be reqiiired which not merely anchors 

 but also acts on the food molecule to make it more readily assimi- 

 lable. These two types are shown in A and B respectively (Plate 2). 

 It will be noted that the receptor in B possesses an anchoring group 

 {h) and an active group (Z) which acts on the food molecule. It 

 is conceivable that an economy in structure could be effected in B, 

 if, in place of the active group (Z), there were merely provision for 

 the anchoring of an enzyme. The active group (Z) could then be 

 dispensed with and the enzyme called upon only when a food mole- 

 cule had been anchored by the receptor. Such an arrangement is 

 shown in C (Plate 2). 



Weigert's " over prodtiction " theory. At this point you may 

 very properly inquire why we assume the existence of receptors of 

 these types. To explain this, let us go back to the productiön of an- 

 titoxin in response to injections of toxin. It will be recalled that 

 the toxin can be neutralized by the antitoxin. Moreover, and this 

 is the important point, this action is strictly specific, so that, for 

 example, diphtheria antitoxin neutralizes only diphtheria toxin; 

 against any other toxin it is absolutely without effect. Since it 

 can be satisfactorily shown that the antitoxin is not altered toxin, 

 it is necessary to explain the productiön of antitoxin by the body 

 cells. We have said above that only such substances can serve as 

 nutriment for the cell which can be tied chemically to the cell pro- 

 toplasm. Expressing this in terms of receptors, we would say that 

 only such substances as possess groups fitting the receptors of the 

 cell can be anchored to the cell. In thinking of these groups and the 

 way in which they fit together, we must have stereochemical rela- 

 tions in mind. Ehrlich cites with approval a simile used by Emil 

 Fischer, saying that the relation of the two groups must be that of 

 lock and key. Granted, now, that certain food molecules have been 

 anchored by fitting cell receptors, what follows? To explain this. 



