114 PATHOLOGY 



materials for the advancement of medical science along new lines. 

 Henle had anticipated many of our ideas of the interaction of para- 

 site and host, but especially interesting are the teachings of Bre- 

 tonneau in regard to the specificness of infectious processes, and the 

 words of his pupil, the great Trousseau, have proved themselves of 

 prophetic significance: "There are [in infectious diseases] two fac- 

 tors; one is the morbific germ coming from without, and the other 

 is the economy about to receive it; there is required a special apti- 

 tude for the organism to respond to the action of the stimulus . . . 

 when there is no such predisposition the morbific germ perishes." 

 It was necessary to erect the great structure of cellular pathology, 

 and to make brilliant and epochal discoveries in morbific etiology 

 before the suggestions in Trousseau's statement as to the interaction 

 of host and parasite could be expressed in such definite terms, and 

 given such enlargement in scope as in the genial and heuristic side- 

 chain theory of Ehrlich. According to this theory a toxin is poison- 

 ous only when it unites chemically with some constituent in the cell 

 of corresponding stereochemical configuration. If the cell does not 

 contain this particular constituent the toxin is harmless; and when 

 these constituents course in the blood as the result of reproductive 

 processes in the cells they are protective antitoxic because 

 they unite with the toxin and thus prevent the disastrous union of 

 toxin with cells. In other words, the substance in the body which, 

 when situated in the cells, is a primary essential for the toxic process, 

 becomes a curative agent when it enters the blood-stream (Behring). 



Fortunately for the therapy and prevention of diphtheria, tetanus, 

 and a few other essentially toxic infections, these antitoxins may 

 be caused to accumulate in large quantities in the blood of certain 

 animals when artificially immunized by the injection of increasing 

 doses of the corresponding toxin. It was a happy inspiration indeed 

 that led Behring to use the antitoxic serum of immunized animals 

 for curative and prophylactic purposes, thus turning to the common 

 good this innate faculty of the animal organism to develop in so 

 marvelous a manner its own resources. 



Supported by numerous experiments among the most imagina- 

 tive and interesting of modern biologic investigation, Ehrlich's 

 theory has proven a veritable master-key to some of the innermost 

 secrets of toxic and antitoxic action and immunity in general. The 

 theory has been found adaptable to other closely related problems 

 in chemical biology, and its signal usefulness in promoting investi- 

 gation in this complex field upon broad comparative basis places 

 it among the great theories of science. 



Ehrlich's side-chain theory has been applied with great success 

 to the explanation of the formation by cells, and also of the action 

 of the various lytic or solvent substances for animal cells, particu- 



