118 THE SIDE-CHAIN THEORY 



to the combining group of the food molecule, but in addition, either 

 a second group of ferment character as part and parcel of the same 

 receptor, or a second haptophoric group which might anchor ferment 

 molecules, normally occurring free in the blood, when the other 

 combining group is occupied by a food molecule of a certain structure. 



Experimental investigation has shown that receptors of both types 

 actually exist, and we may accordingly conclude that the antigens 

 in question, like the toxins, do not represent true foodstuffs, but 

 become anchored to the cells only because they happen to possess 

 haptophoric groups which are identical in structure with those of 

 the normal foodstuffs that the particular receptors are in the habit 

 of binding. The consequence is that here, also, the cell will cast 

 off the useless receptors and produce the same kind in unnecessarily 

 large numbers, the excess being thrown off as in the case of the anti- 

 toxins. If, then, such free receptors meet with their respective anti- 

 gens an interaction between the two will occur and this interaction 

 will manifest itself by agglutination, precipitation, lysis, susceptibility 

 on the part of a given cell to phagocytosis, etc., as the case may be. 

 We should bear in mind, however, that the result, whatever it may 

 be, cannot be viewed as being due to an interaction between the 

 antibody and antigen as a whole, but as the antibody production 

 is, no doubt, the outcome of the presence in the antigen of a definite 

 molecular complex, the visible effect is merely the expression of an 

 interaction between the antibody, and that particular group; agglu- 

 tination, precipitation, and cellular lysis are thus purely secondary 

 results. 



In our illustration of the diversity of receptors which we conceive 

 to exist in a given cell (Plate I), we have designated the essential 

 differences in their general character by differences in color, and have 

 assumed that receptors of one color can only combine with food 

 molecules of the same color. The difference in the structure of the 

 receptors is, however, best shown according to Ehrlich's original 

 schema (see Plate II). 



According to this diagram, then, we recognize three different kinds 

 of receptors or haptins, as they are also called. Those of the first 

 order possess only a single combining or haptophoric group, by which 

 they unite with a corresponding group of the respective antigens. 

 The antitoxins, antiferments, tropins, and anticomplements belong 

 to' this category. 



