THE IMMUNITY UNIT FOR STANDARDIZING 
DIPHTHERIA ANTITOXIN. 
[Based on Ehrlich’s normal serum. Official standard prepared under the act approved July 1, 1902.] 
By Milton J. Rosenau, 
Passed Assistant Surgeon, Director Hygienic Laboratory, Public Health and Marine- 
Hospital Service. 
EHRLICH’S SIDE-CHAIN THEORY OF IMMUNITY. 
Ehrlich’s^' side-chain theory is a brilliant chemical conception, giv- 
ing the onl}" satisfactory explanation of many of the phenomena con- 
cerned in immunity. In one sense it has been likened to Weigert’s 
teachings of inflammation and the process of repair in so far that cog- 
nizance is taken of nature’s prodigality. For instance, a much larger 
amount of material is thrown out than necessary to repair a wound. 
So, too, in antitoxic immunity a much larger amount of antitoxin is 
produced than necessary to neutralize the toxine. 
In Ehrlich’s conception the fundamental processes of immunity 
reside in the cells of the body. These cells are attacked by the poison 
and if not destroyed are stimulated into the overproduction of “anti- 
bodies” capable of combining with and neutralizing the poison. 
Just what cells of the body play the most important role in the 
production of immunity is not exactly clear. It may be, as Ehrlich 
supposes, that this power resides in an}^ organ or tissue. 
In Metchnikofi’s^ theory of phagoc}Tosis the free ameboid cells of 
the body play the all-important role. It is the macrophages and 
microphages seeking food that engulf and destro}" the bacteria. Iji 
the process of digesting the bacteria the cell secretes the enzymelike 
substance which has the power of neutralizing the bacterial poisons. 
It is surprising that the nutrition and metabolism of the cell lie at 
the foundation of two such divergent theories as Metchnikotf’s and 
Ehrlich’s. 
« Ehrlich: Die wertbemessiing des diphtherieheilserums nnd deren theoretische 
grundlagen. Klin, jahrb., Jena, v. 6 (2), 1897, pp. 299-326. 
TJeber die constitution des diphtheriegiftes. Dent. med. woch., Leipzig, v. 
24 (38), 1898, pp. 597-600. 
Croonian lecture. On immunity with special reference to cell life. Proc. 
roy. soc., London, v. 66, pp. 424-448, pis. 6-7. 
^Metchnikoff, Elie: L’immunite dans les maladies infectieuses. Paris, 1901. 45 
figs., 600 pp. 
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