ANTITOXIC IMMUNITY. 179 



But suppose the damage to be but slight, as may be the case in artificial immuniza- 

 tion or in early stages of an infection. The cell is at least deprived of the useful offices 

 of the receptor to which the toxin molecule is now united. This is in itself a loss to the 

 cell, and through the regenerative impulses common to all living cells it proceeds to 

 regenerate the lost parts, which in this case are the receptors. But as more receptors 

 are thrown out of function by the continued action of the toxin, the necessity for com- 

 pensation continues. 



Now it is a fact long known to pathologists, and especially emphasized by Weigert, 

 that the regenerative impulse is apt to be in excess of the obvious requirements and 

 leads to overproduction of new cells, tissues, chemical substances, etc. This is what 

 now happens to the cells, some of whose receptors have been rendered useless by com- 

 bination with the toxin. New receptors are formed, more than the cell requires; so 

 numerous may these become that many are at last cast off into the blood. Here is the 

 point at last. These receptors or substances, normal and useful to the body, but now formed 

 in excess through over-compensation, and set free from tlie cells into the body fluids, are the 

 antitoxin. 



For, set free in the body fluids, these superfluous receptors still retain their com- 

 bining power for the free toxin molecules, which, also, are in solution in the body fluids, 

 and unite with them. This union having been effected, the toxin molecules are no 

 longer a menace to the cell, because the affinities are now satisfied through which they 

 joined the receptors while these were still a part of the cell, and in this way became 

 harmful. This now inert combination of toxin molecules and detached receptors is 

 physiologically indifferent stuff, and may be removed from the body by the usual proc- 

 esses of excretion. 



But the antitoxin which has not united with toxin in the body of the animal 

 which produced it is still available on the transference of the serum to another in 

 dividual whose blood contains diphtheria toxin, and who may thus secure passive 

 immunity. 



If we have recourse again to the graphic method, the hyperproduction of recep- 

 tors by the damaged cell, their separation, and their action as antitoxin, may be indi- 

 cated, as in Fig. 89. Let A represent a portion of a cell body. The toxic molecules 

 a, uniting with the receptors b, lead through the injury to the cell, as well as by its 

 deprivation of the normal use of b, to the production, and at length to the overproduc- 

 tion of new receptors of the same kind. These superfluous receptors c are now set free 

 into the body fluids, 1 where, as at x, they may freely unite with the toxin molecules, 

 forming harmless compounds and preventing further access of the toxin to the cell, 

 where alone the damage can be done. Or, when free, as at y, the receptors may be 

 transferred in the serum, becoming effective as antitoxin in another individual. 



It is well to note that so long as the receptor maintains its connection with the cell 

 it is not antitoxin, but an element of vulnerability to the cell. It is only when the 

 receptor has been set free from the cell that it is antitoxin. 



The antitoxic substance which neutralizes the action of the toxin molecule of diph- 

 theria is not effective for the poison of tetanus, for example, simply because it does not 

 combine with the molecule of tetanus toxin. It is specific for diphtheria, because it 

 was the diphtheria toxin which excited its overproduction through a chemical union 

 identical in character, whether this union takes place while the receptor is a part of the 

 cell, in which case the toxin becomes harmful, or when the receptor is detached from 

 the cell, in which case the combination is harmless. 



In the light of this hypothesis the specific character of the antitdxic substances 

 appears to be but the result of adaptation to unusual conditions of cell capacities evolved 

 and fostered for the every -day maintenance of life. The specific relationship between 

 the toxin and the antitoxin is not developed during immunization, but existed before- 

 hand as a necessary condition of toxic action. 



This hypothesis not only accounts for the formation, protective action, and speci- 



1 These receptors of various kinds, cast off into the body fluids under the most di- 

 verse conditions, Ehrlich has called "haptines." 



