176 ANTITOXIC IMMUNITY. 



blood of the artificially immunized animal contains a substance, or sub- 

 stances, called diphtheria antitoxin, which, on being introduced with the 

 blood serum into other susceptible animals, may not only confer a quickly 

 established immunity passive immunity but, without destroying the 

 diphtheria germ, may protect against its toxic effects when the disease is 

 already under way. Thus through the artificial immunization of horses 

 and the hypodermatic use of the serum of their blood in man the so-called 

 "serum therapy" has assumed a very important and beneficent role in 

 the prevention and treatment of diphtheria. ' 



Besults similar to those obtained in the study of diphtheria antitoxin 

 have been realized in the investigations of tetanus. But the practical 

 value of the tetanus antitoxin as a therapeutic agent is less obvious. It 

 has been shown further that antitoxic sera may be developed in the body 

 during immunization to snake venom, vegetable poisons, such as ricin, 

 and to other albuminous animal and vegetable materials. Thus, also, 

 through experimental adaptation, the living body cells may elaborate and 

 set free into the serum substances which suspend the action of ferments, 

 like rennet, pancreatin, fibrin ferment, etc. 



Not all toxic substances, however, are capable of inciting the living 

 body to the formation of antitoxic substances ; nor are these apparently 

 formed in such considerable amount in most of the infectious diseases, 

 as is the case in diphtheria and tetanus. 



The action of these antitoxic substances is, within certain limits, spe- 

 cific; that is, the antitoxin of diphtheria protects against diphtheria, 

 that of tetanus against tetanus, etc. 2 



EHRLICH'S "SIDE-CHAIN" HYPOTHESIS. 



We have seen that a most remarkable series of facts is developed by our studies of 

 infectious diseases illuminated by the results of animal experimentation. When the 

 living body is invaded by a certain toxic complex organic material, not always of mi- 

 crobic origin, the body adapts itself to the new conditions by the elaboration of sub- 

 stances which protect it from the action of that poison. Each new protective substance 

 is effective against the particular poison which induced its formation. Not only this, 

 but if the blood serum containing these new protective substances be transferred to 

 another individual they protect him also passive immunity against the special poison 

 which called them forth in the body of the first. 



How are we to conceive of this wonderful adaptive power of the living body under 

 conditions which seem to be wholly new in the life of the individual; a power which 

 at first appears to transcend the known capacities of the body cells? 



1 The antitoxic value or power of each specimen of antitoxic serum is experimentally 

 determined by finding the amount of the serum required to protect the test animal 

 against the action of a definite amount of toxin of known strength. Various standards 

 have been adopted, the value of the serum being expressed in terms of the "anti- 

 toxin unit." For example, in the determination of antitoxin value made use of by the 

 Department of Health of the City of New York, an antitoxin unit is the amount of 

 antitoxic serum required to protect a guinea-pig weighing 250 grammes from death, 

 when one hundred times the fatal dose of diphtheria toxin is mixed with the serum and 

 the mixture injected subcutaneously into the animal. The usual dose for human ad- 

 ministration may contain from three hundred to six thousand units. 



2 For a detailed consideration of the specificity of infectious agents see Kolle in 

 Kolle and Wassermann's "Handbuch der Mikroorganismen," Bd. i., p. 288. 



