ANTITOXIC IMMUNITY. 177 



This singular capacity of the body cells to develop in emergencies apparently new 

 substances has naturally given rise to much study and to many speculations and hy- 

 potheses. The scope of this book does not permit even an enumeration of these. But 

 it is necessary to bring forward at least in outline the so-called " side chain " hypothesis 

 of Ehrlich, by which he strove to account for the phenomena of antitoxic immunity as 

 exemplified with especial clearness in diphtheria and tetanus. Without this hypothesis 

 we are to-day still practically at sea in our views of the nature of antitoxic immunity, 

 while tin's remarkable capacity of the body to manufacture the greatest variety of 

 potent specific antidotes to the most subtle and virulent poisons becomes, for the first 

 time, comprehensible in the light of this ingenious conception. It is now several years 

 since Ehrlich's hypothesis was enunciated, and whatever may be its merit as a direct 

 contribution to science, there can be no doubt that it has been, in a remarkable degree, 

 an inspiration to the most fruitful research. It is practical!}' impossible to follow the 

 recent work on immunity without a knowledge of this hypothesis and familiarity with 

 the nomenclature to which it has given rise. 



In order to comprehend the hypothesis of Ehrlich regarding the origin and nature 

 of antitoxin, it is necessary for us to form with him a clear conception of cell assimila- 

 tion. 



We conceive of the cell as a mechanism for the storage of energy derived from 

 without and for its release under definite conditions. This storage of energ3 r is possible 

 through the assimilation and building up by the cell of complex molecular combina- 

 tions. These, owing to their instability, are readily resolved into less complex and 

 more stable combinations with the release of the stored-up energy. Thus is the life of 

 the cell manifested. 



This broad conception of the cell is a purely intellectual one, however, for the de- 

 tails of cell metabolism still elude the keenest scrutiny of the chemist. He cannot for- 

 mulate protoplasm nor express its chemical changes by proportionate symbols. 



Now, accepting this condition of affairs as for the moment inevitable, Ehrlich 

 seeks to express his view of the character of the cell's performances in general terms, 

 disregarding its morphological peculiarities, somewhat as follows: We may conceive of 

 the cell as consisting of a central group of very complex molecular combinations which 

 maintains the characteristics and special capacities of the cell as an organism under all 

 the vicissitudes of its existence. Associated with this central organic group are many 

 and varied subsidiary atom-complexes which by means of their unsatisfied affinities 

 bring the central group into relationship with food material through those chemical 

 combinations which in living protoplasm characterize assimilation. 



These unsatisfied affinities by which assimilable material is fixed or united to the 

 cell have been called "side chains," a term adopted from the chemist. Not to press too 

 closely, however, the analogy between the chemical processes in lifeless substances and 

 assimilation in living matter, these affinities or " side chains " of protoplasm are now 

 commonly called receptors. 



If we seek to illustrate Ehrlich's conception we shall be obliged to use graphic 

 figures of extreme crudity. If the arc of a circle in Fig. 87 represents a portion of the 

 periphery of a cell, we may indicate the side chains or affinities or receptors by projec- 

 tions whose special shape shall indicate their special capacity to combine with any 

 substance coming in contact with them under favorable conditions. Suppose in this 

 figure we let a represent a nutrient molecule which is capable of combining with the 

 receptor b, belonging to the cell A Through its union with b, and only through this, 

 is it capable of entering into the metabolism of the cell. This moleculfe a cannot unite 

 with the receptor c or d, but only with such receptors to use the crude expression 

 which our illustration requires as it fits. Through the receptors c and d other forms 

 of food molecules may enter into the metabolism of the cell. 



Thus it is in Ehrlich's conception, which after all is only a graphic way of illustrat- 

 ing the preliminary phases of assimilation by living protoplasm, that the cell is capable 

 of selecting or " fixing " out of the host of various substances with which it comes in 

 contact, just those and only those to which its receptors bear a definite chemical rela- 

 tionship 



12 



